Now he's back to play the title role in the first revival of David McVicar's production of The Marriage of Figaro.

It opens next week with a mostly new cast and a different conductor, renowned Mozartian and former Royal Opera Music Director, Sir Colin Davis.

I meet up with Ketelsen on the hottest day of the year so far, journeying through the seemingly ten miles of backstage at the House and arrive perspiring, to find him sitting relaxed in shorts and reading a novel. 'It's such a warm day', I remark. 'True', he says. 'But it's better than rain!'

So after Henrik, Ketelsen is back with another operatic manservant, perhaps the most famous of all – Figaro. How different are these two characters?

'They're similar in some ways, but they're a bit different. I think Henrik is a bit more openly insolent, quite so; Figaro…is quite a jealous spouse, and I think that's where he gets into trouble. They're similar because they're both kind of revolutionary, and Henrik is fashioned after Figaro a bit. Figaro is sort of mischievous with a plan, whereas Henrik's head is elsewhere, he's sort of mischievous for his own benefit.'

David McVicar's production is set in the 1830s in an elegant chateau, and it's full of detail and stage business. Does our new Figaro find it compelling?

'Yes I do, I like it very much. Business is fun as long as it deals with the plot. And I like very much all the servants and actors we have in the show. As was explained to me, it matches perfectly to the movie Gosford Park, if you've seen that? The behind-the-scenes dealings and relationships between the servants, and especially the servants and the masters. You know, with operas such as Figaro and others that are done so much and have been done in an infinite number of ways, directors run out of things to do here and there, and he really brings a new batch of ideas to the mix, that really make sense, especially in scene changes.'

Colin Davis has been conducting Figaro and the Mozart operas for decades, as well as recording the major works at least once. What's it like working with this legendary figure from the world of opera?

'Well, I try to suck everything up he says, like a sponge. The more you do this, the more you realise how little you know and hopefully you keep an open mind and try and learn a little more from every person that you work with…Some people take the attitude, 'I know it all, why would I want to listen to you?', but it's a double-edged sword. It makes you look ridiculous, people think 'Why is he acting like that, why doesn't he take advice?' And on the other hand, it limits you as a performer. The more you listen and take it in, the better you get, hopefully, ideally - that's the point, right? To enrich and grow personally as an artist.'

Davis' enormous knowledge of the music he conducts always brings new insights – has anything stood out?

'He's mentioned a whole bunch of things…probably ten or fifteen things that come to mind, if I went through the score I could say, 'Here he points out this' – and it makes sense, dramatically. He'll say, 'come in a little early here because you're arguing with the Count, and if you leave that space as written, it's very nice and metric, yes, but it's vague and doesn't have any passion behind it.' When I was at university – I think I was 19 – I wrote an essay on him, never thinking I'd be here singing at Covent Garden with him as the maestro. So it's all very humbling. You know, think about all the people he's worked with, for how many years – he still enjoys it, he's passionate about it. He's been a wealth of knowledge.'

For the first revival of Figaro, the Royal Opera has brought together an even more starry cast than we heard in January. Soile Isokoski and Michael Volle play the Almavivas; even the veterans Helen Donath and Robert Lloyd are making cameo appearances. But Figaro and his bride are being played by suitably young singers - Ketelsen, of course, and Isabel Bayrakdarian as Susanna. What's the atmosphere like in this amazing ensemble?

'Bob Lloyd, Robert Lloyd – there's another one, I've been asking him questions left and right. To share the stage with someone like that is just amazing.

'It's good to have young people in Figaro. It's tough to always do. But we're quite a young cast – I'm 34, I think Isabel's 33, something like that. For opera, that's very young.

'The first time I met Isabel, we were both in Plàcido Domingo's competition, Operalia - it was in Los Angles. She won first prize, and I was one of the finalists. And then we worked together in Chicago as Masetto and Zerlina in Don Giovanni.

'A couple of days into rehearsal [for Figaro], I'd been sick with a sinus infection for just over two weeks, so we weren't touching a lot or kissing a lot, you know. But we're supposed to get intimate, and Stefan at one point said, 'You're gonna have to get intimate and get to know each other at some point!' and I said, 'Ah, we're old pals. I mean, I've had my hands on her breasts when we were in Chicago, so don't worry about it, we'll get back to it right like that!'

Mozart is the backbone of Ketelsen's repertoire. Forthcoming appearances include more Figaros in Barcelona and Boston, and Leporello in Don Giovanni, including the Covent Garden revival in 2008. Is Mozart still a great challenge to sing, or is it comfortable?

'When I was younger, when I was starting out, still studying, I always thought Mozart was very tough. Some of it is, for me. It's quite a common opinion that Mozart is the most healthy thing for a singer, it's the most healthy activity for a singer to do. I don't know why it is – it's just the way he wrote. I'm not sure he did it purposely, it's just that his style of writing happens to be a healthy way to produce sound, nothing too driving, it doesn't ask too much of the singer – it's not like Wagner where you should be in your 40s when you start. You could start with Mozart when you're very young and you'll never go wrong.'

'I've sung Guglielmo before, in Cosí fan tutte, and that wasn't comfortable, but that was just a Fach issue, it was just a little too high. But typically, my meat and potatoes Mozart is Leporello in Don Giovanni and Figaro in Marriage of Figaro.

In the last two years alone, Ketelsen has sung three different roles in Don Giovanni - the Don himself, his servant Leporello, and Masetto. What's it like singing all three roles?

'Ah, I've actually done the four of them! The first time I did Don Giovanni was as the Commendatore in English and I was 22 at the time. Then I did Giovanni in English in graduate school, which was tough, that was really tough. I've done a lot of Leporellos since then and I've just done my first 'real' Giovanni in Italian, in Minneapolis. It was enjoyable, though I think I prefer Leporello – and then there was Masetto in Chicago with Bryn and Isabel….Singing all 4 parts, you really get to know the show. And a lot of singers have done that – you can just do an entire scene by yourself – sing Giovanni, Leporello, sing Masetto. It really helps you in the sense that you know what the other person is saying, and it gives you a better reaction to that, though ideally you should know it anyway. It gives you a better overall picture and feel for the piece.'

Having made a name for himself as Figaro and Leporello, does Ketelsen have any plans for further Mozart roles?

'I was asked to do a Papageno recently, but I turned it down because it's not me. It's just a baritone type of role. And Guglielmo in Così – I sang it, I sang every note, but I'm just not a baritone. I have the higher notes when I need them, but I'll leave it to the people who are naturally that Fach. As far as Sarastro's concerned, I don't think I'll be asked – I can sing all the notes of that too, but people want big and woofy sounds, which is not me, I've more of a bright sound.' And you have to be eighty years old, I suggest. 'Exactly! Maybe after I've been retired for twenty years!'

Nevertheless, Ketelsen has further Mozart this year in the form of concerts. 'It's a big Mozart year, so everybody's doing it. At the Proms with Roger Norrington, I'm doing the finale of Don Giovanni, and Mozart opera highlights with John Eliot Gardiner, though I'm not exactly sure what I'm singing yet. And I've got a number of Figaros and Leporellos, including here.'

Ketelsen is such a laidback figure that I wonder, did he always want to be a singer? 'No it wasn't always an ambition. I have always sung, according to my parents I've sung since I was a baby. Whether it was on pitch or not is another issue, but I sang in school choirs when I was growing up, right all the way through school…I did competitions at high school and did well. We had a prominent music programme in our home town, and the choir director saw some potential in me and encouraged me to do the competitions.

'When I first went to college, I didn't know what I wanted to do – I actually wanted to go to the army and fly helicopters, so I joined the National Guard and I was in that for six years while I went to college…I was two years in a private college and still had no idea what I wanted to do, nothing interested me. Then I transferred to the state college, the University of Iowa, and I still wanted to sing, still wanted to take lessons to keep my voice up because I'd always been told 'you should sing, it's a good voice, blah blah blah'.

'Then the man who became my voice teacher for the next four years said, 'I think you should become a voice major' and I said 'OK, whatever, it's as good as anything else. So I did that, and slowly but surely I figured out that I was actually good at it and I could actually make a living out of it. So I went from there and went to grad school and studied with a man who was famous in the 50s, 60s, and 70s, named Giorgio Tozzi. He sang here a number of times.'

And on the soundtrack of the movie of South Pacific, I interject. 'There you go! That's not Rossano Brazzi singing, it's him singing 'Some Enchanted Evening' [he demonstrates]. 'Most people don't know that, very good!'

'I went to Indiana University grad school – it was a much bigger pond, it's the biggest music school in the world. I did well there and thought, OK, I can do this for a living. From there, I got an agent while I was still at school, and when I was done at school things just started happening. I did lots of competitions, and I did well, made a lot of money.'

Did training for all those years help to prepare him for the life of an opera singer in the real world? 'It's to your advantage to study longer because most voices take time to mature…It's not like the business world where you do your degree then suddenly you're ready for the whole world when you're 24. I was finished with school, when I was 27, and that's pretty young. A lot of people don't get a start until they're my age now, early 30s, and that's fine. You're really considered a baby until you're 40. It is an advantage in my opinion.

'For me it worked out perfectly. I found the perfect teacher, I studied with him for 4 years. At that point in my learning, which was nil, he was great: I learned technique, diction, languages, and then I switched schools and started studying with not as technical a teacher, someone who understood repertoire…I learned so much in all this time, and got a lot of performance experience. So by the time I was released on the world, I had been basically singing for a professional opera company, which was Indiana University, because they run it like an opera company, with their own venue, a 1500-seat theatre, their own costume shop, and their own sets, and it's very impressive. You have rehearsals there like you do in the real world.'

Out in the real world, on the international opera circuit, he's already worked with some of the world's great conductors. Who stands out as a singer's conductor? 'David Robertson in St Louis…He's just great. First of all, most importantly for me, he's just a regular nice guy. Is that too much to ask? Just a normal person! Sometimes it feels like they're hard to find…He understands what the voice is doing. He's just low-key, he's cool with it, open to suggestions, understands the whole idea that it's a collaboration – image that!

'And Colin, Sir Colin – I'm not just saying this – he's been around so long that he must be doing something right. He knows what it's like to be a singer, he's been around. And he's equally low-key. If he has a suggestion, it's for the better of the performance and the piece as a whole. Ego has long since taken a back seat. So I really enjoyed working with him….And Charles Mackerras was very good too – you just sit back and listen and wait for the next gem.'

He recently made his Met debut as Angelotti – 'It wasn't my first time on stage, though. I did their competition in 1998, I was a winner – so at least I knew what it was like (not that that made for fewer butterflies on opening night!)' and now he's back at the Royal Opera House. How does working here compare to America?

'Just from a cultural standpoint, it's more ingrained in European culture, obviously, it's the birthplace of opera, Italy, France, Germany. It's more in your blood than it is in ours. There are a comparable amount of people who are big opera fans. But I'm not so sure that BP big screens would work over there. I love it – I wish it were like that in the States. Here, the public is more knowledgeable on the whole about opera.'

And what's it like working for the Royal Opera? 'Everybody's just so nice, and so eager to please. You try and reciprocate; hopefully it's a good relationship. It's that way in Chicago too, it's that way in New York, though the Met is quite a machine.'

Not only in demand in the opera house, Ketelsen is giving increasingly more concert performances. As well as the Mozart concerts mentioned above, he returns to London with the Monteverdi Orchestra next year in Haydn's The Seasons, and appears with the National Orchestra of Paris later in the year. How different are the experiences of singing in concerts and singing in operas?

'From a performance standpoint, I enjoy concerts very much. But I think operas are easier for me. In concerts, you typically wear a tuxedo, tails, tie, there's no kneeling on the floor, hugging, kissing, flying through the air….But even in concerts, you have to put a little character in it. Even in straight-laced Messiah, you need to be the voice of God or you need to be Adam in The Creation. Some concerts, like I'm doing the Berlioz Romeo and Juliet in Paris with Colin [Davis], and obviously, I'll be a character. So I can put a bit more into it.'

How about Lied recitals? 'It's quite intimidating to me, although one accompanist said to me once, even if you're not in the best of voice, just communicate with them, tell a story – and after that, it was like a switch was flicked, and it was so much easier.' He has no definite plans at the moment, though, saying that there's no demand for Lieder recitals.

What's his attitude to new music? 'Contemporary music – it's not my favourite, but I'm open to it.' Does he fancy having a role written around him? 'Oh gee, boy, that'd be rough! No, course I'd take that. Maybe I'd want to have some imput, 'Can I have a line here, that you can walk away humming?!' Sure, how flattering is that, course I'd take it!'

With opera audiences ageing daily, how does Ketelsen see the future of opera?

'With cautious optimism. A lot of people are working on it to keep opera relevant. I'm a big fan of outreach, especially for kids. Because when you grow up you go to things which make you feel at home, that make you comfortable. And if you were exposed to opera as a child, you're more likely to enjoy it. So I think outreach is a big part of it. School music programmes are a great part of it. Exposure, so that people don't have the misconceptions that they often do. You know, on the radio a couple of weeks ago at home, there was a talk show with four hosts. They were talking about opera and this woman said, 'Ah, opera, I do not like opera. Well, I've never actually been to one, but I can tell you right now that I don't like it'.

'I can't tell you how many times I've brought new people to a show – on my travels I see lots of old friends or relatives, and I try and expose them to it. And you'd be amazed how many are surprised, they say, 'I never knew' – especially operas like Figaro or Carmen - 'I knew three numbers in that, I can hum them'. And the stories, it's like a play set to music, it makes sense. There's comedy and there's drama. People are worried that they won't understand it and I say, 'There's supertitles!' So like I say, I have guarded optimism, I am more optimistic in Europe than I am in the States, because people here know more than they do there. And you might be thinking, that's not much – well, it's even less in the States.'

At 34, Ketelsen is already clearly one of the great figures of opera in the future. What aspirations or plans does he have? (You're allowed to be normal, I tell him!)

'I just hope that I'm part of my kids' lives enough, and home enough – it's tough. I've got to the point where I say no a little bit more, and say 'no, no, no, this amount of time is set aside when I'm going to be at home, because nothing is more important to me than that.

'As far as music is concerned, in general, I want to sing in all the major houses in the world, when I want to – not too much! I guess I would like to be one of the names that comes up, when people say 'who do you want for this role? This part would suit Kyle!' – that would be nice. And it would be nice to leave your mark on something after you've gone, too - for my kids to look back in 50 years' time and say, 'Wow, my dad left his mark, he did this', and for someone to come along to my kids and say, 'I knew your dad, he was a really nice guy'. I think that would mean a lot to them – and me.'