Some actors rise effortlessly through the ranks, going from supporting roles to leading parts in the space of a year or two. Others rack up credit after credit without ever making the transition. For Dermot Mulroney it's been a case of the latter. He was one of the outlaws in Young Guns and made an early impact playing an AIDS patient in Longtime Companion but too often found himself appearing in projects that sounded promising but ended up being dire. Think River Phoenix's final film The Thing Called Love or Bad Girls. There have been a few gems along the way - including Kansas City for Altman and the melancholy Western Silent Tongue, again with Phoenix - but none of them have been commercially successful. Now, after almsot ten years, he's got his chance to step up with next month's smash romantic comedy My Best Friend's Wedding.

As the best friend of the titel, he plays a sports writer who has to choose between getting hitched to fellow journo Julia Roberts or multi-millionaire's daughter Cameron Diaz. A tough choice for anyone, even if Mulroney acknowledges that only in the movies could a low-paid hack come within a mile of scoring with either.

"There must be something extremely appealing and charming about Michael," he laughs. "One of the grips said, 'It looks like Michael fell with his ass in the butter.'"

Despite turning in a charming performance opposite Roberts - at her most beguiling - the 33-year-old Mulroney is still hedging his bets about whether this really is his breakthrough part.

"I've been sort of thinking about," he concedes, "but I'm not concerned. If I'm invisible after this film it'll just be like it is today and I've got nothing to complain about. What I mean is, 'Yeah, probably, but who cares?' That's the safe way to approach it for me."

The affable and quick-to-smile Mulroney already has recognition problems; he's spent much of his career being confused with Dylan McDermott.

"I don't know if it's ever happened before in Hollywood, where two guys are mixed up like that, so at least it's a unique experience and we both get a kick out of it."

Even though Mulroney is the man in the middle, My Best Friend's Wedding is very much a vehicle for Roberts and working with someone who is a genuine phenomenon is not like acting with anyone else.

"It's weird, I suppose, but I've learnt from the past not to let what you think of somebody else affect your job. My job isn't to sit on the set going, 'Wow, that's Julia!' because I did that with Richard Harris and Alan Bates (on Silent Tongue). You're wasting your time because you should be thinking about acting. I actually made a conscious effort just to approach it like I do any other job and I'm really glad I did because that's how Julia approached it, that's how Cameron approached it."

That the film avoids being another predictable entry in a genre that's been poorly served by Hollywood is due to both the performances and the way Australian director P.J. Hogan, who was responsible for the excellent Muriel's Wedding, approached the material. Mulroney first met the director when he worked with Hogan's wife, Jocelyn Moorhouse, on her 1995 American debut How To Make an American Quilt.

"I don't know any other couples that are both dirctors. It's a really rare combination. I just have immense respect for how they even get through a day," says Mulroney. "Obviously, P.J.'s got a little wackier edge to his films; that's as much to do with him as it is to do with the material he's chosen."

That emerges in My Best Friend's Wedding during one sublime scene that must count as the best use of helium balloons ever in a movie. At the smae time thoguh, the film asks us to believe that a man can be best friends with his ex.

"It's funny, that have never occurred to me. Maybe I just took the script at face value. I also feel like I married my best friend so that issue doesn't exist for me in my life...I think it's probably possible; seems like it should be."

Mulroney's wife is actress Catherine Keener, who's best known for playing lead in Living in Oblivion, a film in which he also appeared as a gloriously obnoxoius cinematographer. In fact, he had liked the script so much that he even put up a portion of the budget.

"If I could repeat that each time I'd do that over making money, having a big dressing room and all that any day of the week. Steve Buscemi does that beautifully; he slips into Con Air then comes back and directs, wirtes and receives accolades. I'm proud to count him as a dear friend."

Growing up in the suburbs surrounding Washington DC, Mulroney's initial ambitions rovolved around music and it was only when he reached North Western University that he started to act. He still plays cello in the Low and Sweet Orchestra, a punk-folk outfit who released an album last year. He'll next be seen in Roland Joffe's Goodbye Lover, a dark, twisted comedy in which he costars with Patricia Arquette.

"I like movies that are just a little screwy, they've got something that's a bit different. It's a true dark comedy in that it may take a couple of reels for the audience to work out this is funny...I have really high expectations."