They make for an odd couple, Colin Firth and Stellan Skarsgard. At least, so I thought until I met them together.

The former is something of a reluctant hearthrob, melting a million hearts in the BBC's 1995 TV production of Pride & Prejudice, and neatly repeating the trick in 2001's Bridget Jones's Diary. Skarsgard, on the other hand, is an institution in his native Sweden, and a familiar face over here that you might just recognise from King Arthur or the Pirates Of The Caribbean movies (playing Orlando Bloom's dad).

In Mamma Mia!, each plays a possible father to a young bride-to-be, who has tricked them both, and a third candidate (played by Pierce Brosnan), into coming to the Greek island she lives on with her mum (played by Meryl Streep), in the secret hope of being walked down the aisle by the father she never met.

Firth no doubt got the role because of his dashing good looks and his way with mildly disgusted comic reactions. Skarsgard just happens to be buddies with Benny Andersson and Bjorn Ulvaeus, the Swedish songwriting duo and male half of Abba, whose songs drive Mamma Mia! onwards and upwards.

When I met Firth and Skarsgard in London last week, they were positively giddy. So, I reckoned they could handle a few naughty questions.

PAUL BYRNE: I went in wondering whether Colin Firth and Stellan Skarsgard would actually be able to sing, and I'm still wondering. Did they give you the Ashlee Simpson treatment?

COLIN FIRTH: What's the Ashlee Simpson treatment?

It involves taking your foghorn of a voice and putting it through some digitial filters so it no longer makes babies cry and dogs whimper ...

CF: No, I don't think they did too much of that.

STELLEN SARSGARD: They didn't need good singing, actually.

CF: No, they just needed us. There are plenty of people out there who you could cast in a musical -- people who can really sing -- but they were looking for the right people for the part, not the right voice. When we were filming, I remember going to see Hairspray, and seeing how slick it was, and realising that we weren't going to be doing it that way at all. That wasn't the name of the game here. I mean, I love all that -- the Broadway principle of dazzling brilliance from every single member of the chorus but this one had some people who could sing, and some people who couldn't really.

Colin, you sang before, on The Importance Of Being Earnest and the recent St Trinian's film, whilst Stellan, as a teen sensation in 1960s Sweden, you released an album, but the producer secretly handled vocal duties . . .

CF: This guy knows where the bodies are buried, doesn't he? Usually, you can lie a bit. . .

SS: Alright, alright, I'll talk! I'll talk!

Were you guys nervous, stepping up to the microphone?

SS: Terrified. Terrified.

CF: I don't think I've ever been that nervous. I'd heard that Benny and Bjorn could be hard taskmasters, and I just had an image of those three days, being tortured. I don't know if you saw that documentary about Carreras being bullied by Bernstein, recording West Side Story. It's horrible to watch; Carreras is almost in tears. He delivers an aria that sounds beautiful, and Bernstein takes off his glasses, and he's shaking his head. And Benny was going to be Bernstein, and not that I was going to be Carreras, but I was going to be in floods of tears all the way through. Terrifying.

SS: When I saw the look on your face, I was absolutely terrified. Until I realised that what they wanted from me was the flaws. . .

CF: They wanted three middle-aged, ropey guys who had been exhumed, you know, from the past, and who could give it that personal element.

It must have been easy to tap into the emotional journey of your characters, given that, since you're both handsome, globe-trotting superstars, you both no doubt have kids you don't know about sprinkled all over the world. . .

SS: So true. . .

CF: [Laughs] It's our story. . .

SS: I've been here before; I know what my character's going through right now. . .

CF: Oh, yes, the Mediterranean is littered with my progeny. . .

Was the Mamma Mia! shoot just fun, or was there work involved? It looked like a holiday. . .

SS: Once you threw away your ambitions, and intentions, and stopped having the idea of trying to come off looking well in the film, it was fun.

CF: Yeah, and there are self-help programs out there that try to achieve that.

SS: Everybody should try it.

CF: Stop clinging to that dignity that's holding you back. . .

I know you guys have both shot films in Ireland -- anything outside of the work?

SS: I shot two films there, but I go back every now and then because my fiancee used to live there, and she's got a lot of friends there. I've even been to an Irish wedding. I'm a Swede, I'm strong enough.

Was there drink involved?

SS: I don't remember.

CF: I haven't been for a long time, and I realise now it's been too long a time.

Mamma Mia! hits Irish cinemas July 11

- Paul Byrne