Hey - cut it out, you two!

Paul Gemignani hopes his Sweeney Todd will start a rapprochement for musical and opera

"Musical theatre has separated itself off from opera," says the conductor Paul Gemignani. "But they are essentially the same. For a lot of bad reasons - mostly to do with snobbery - they have walked away from each other, and they need to come together again." This big, bearish New Yorker is doing his bit to ensure that the coming-together takes place: on 15 December, he is presiding over the first performance of Stephen Sondheim's Sweeney Todd at the Royal Opera House.

"Anybody who says we shouldn't be doing this at Covent Garden might as well say we shouldn't be doing Benjamin Britten either. It's a dramatic piece, sung in English like Billy Budd, and it belongs as much in the opera house as in the theatre." The production, he says, will be entirely unlike the celebrated one 10 years ago at the National Theatre. "There wasn't anything wrong with the way Dennis Quilley did it - it was just different."

When asked about the nature of that difference, he explains that it has to do with style. "In a piece like Carmen, the line is purely musical. But here the line is an amalgam of speech and song. Your approach to Carmen is dictated largely by what's on the page. Here, it's dictated much more by the performers you have. The sort of issue that arises in rehearsal is this: do you sing full-voice right away when you come out of the dialogue, or do you ease into it smoothly?"

Casting is crucial. "Some opera singers can't do it, because they're not actors - they're just singers. But Tom Allen sits on both sides of the fence - he's perfect." The multi-talented Allen, whose ambition has always been to star in straight plays as well as opera, is taking on the title role.

Ask Gemignani which other musicals could make the magic transition, and he gives a quick list headed by Candide, Carousel and A Little Night Music. But not Cabaret; not Guys and Dolls. The reason, he says, is simple: "If a piece is not enhanced by 'legit' voices, then it doesn't belong here." So no Lloyd Webber? He sighs and gives a careful answer. "Phantom could work in the Opera House: it's the most legit of all the things Lloyd Webber has written. The rest of his work is basically pop, even if Jesus Christ Superstar is one of the best musicals in the repertoire. Sondheim is a trained classical musician, which is why his pieces work here, but Lloyd Webber is not comfortable in this genre. Two of the songs in Sunset Boulevard are superb, but does he sustain that all evening? No."

Mention Boublil and Schoenberg, and he rolls his eyes. "In Miss Saigon, the first 16 bars of Oriental orchestration are OK, but from that point on, it's all downhill. Their singers don't sing; they belt."

And Mamma Mia!? He looks doubtfully at my tape-recorder, then chooses his words with care: "Everybody has the right to stage something that they think is meaningful."

Then he cracks up with wicked laughter.

'Sweeney Todd', Royal Opera House, London WC1 (020-7304 4000) from 15 December