Keating was one of 7,000 applicants for the part, seven of whom were well-known pop singers. (The producer, Andre Ptaszynski, isn't naming names, but admits that one candidate had been in Neighbours.) Even more remarkably, Keating is only 19 years old - and if I were a barman I wouldn't serve him without ID. We meet at the BBC's rehearsal rooms in west London. He's wearing a nerdy bright-green polo-neck and has a fashionless schoolboy haircut; he's amiable, small and slender, and slightly gawky in his movements - nothing like the Roger Daltrey-ish rock beast I'd imagined. Not only is he hardly the type to throw televisions out of windows, it's a moot point whether he'd be able to lift a TV set at all. "To have someone who looks so pure become a messiah figure adds to the effect," explains Ptaszynski.
Keating has been publicised as an innocent lad plucked from the supermarket shelves and flung into the limelight; in fact, he already has a gilded CV. At 12 he was Gavroche in Les Miserables ("the best year of my life," he says, though 1996 is looking good); at 13 he starred in a BBC drama, The Troublemakers. After that he concentrated on school, but managed to fit in "a couple of radio plays". When he left school, he understudied in Lost in Yonkers, bought a car with his wages, and took the Tesco job so he could afford to run it.
He hadn't heard of Tommy until he saw an advert in the Stage in July 1995 which asked for someone around his age with a "rock tenor" voice. And so began four months of auditions. At most of these was Pete Townshend, The Who's guitarist, who also sits in on rehearsals nearly every day. To Keating, he is just "Pete": "I didn't grow up knowing The Who or Pete," he says matter-of-factly, in a soft East End accent, "so to me he's just a nice bloke who happened to write the show. It's the same with Kim Wilde. She's a lovely gel."
Keating's background is another reason why he got the part, rather than the original Broadway actor. "For all of us there is a sense that this is the musical being seen in its home town for the first time," says Ptaszynski. "That sets up demands of accent and location that don't apply on Broadway. Unlike Ken Russell's film, it's not a picaresque 60s-70s thing; it's a perfectly planted show about growing up after the Second World War. It's as London-based as Oliver!, and you'd never put on Oliver! in the West End with an American cast."
Keating was born at Bart's Hospital, which makes him an official Cockney, and he still lives with his parents in Romford. "I don't know if I'll move, I'll see how it goes," he says, shockingly nice and unpretentious for an ex-child star. "It's good to have a bit of normality. It's gonna be a bit of a culture shock for me. I think I'll need a slap back into place occasionally - and my Mum and Dad'll be pleased to do that." Keating's Mum is a supervisor at Mori, his Dad is a police superintendent. His 22- year-old sister is an assistant producer for Cameron Mackintosh, but Paul seems unaffected by the world of theatre. When asked if he has any role models, he answers no, but to be helpful he nominates Judi Dench, after much brain-racking.
In the last weeks of rehearsals, it's only to be expected that he hasn't thought what he'll do next. But it's unlikely that he'll have to take up Tesco's offer of his old job back. Has he any idea where he'll be in 10 years' time? "Oh God, no. Hopefully I'll have my own place by then." Rock'n'roll!
! 'Tommy': Shaftesbury Theatre, W1 (0171 379 5399), previews from tomorrow, opens on 5 March.
'TOMMY': THE WHAT, WHEN AND WHERE OF THE WHO'S HIT MUSICAL
May 1969: The Who premiere Tommy for the British press at Ronnie Scott's club in London.
June 1969: The album reaches No 2 in the UK charts.
June 1970: The Who play the "final performance" of Tommy at New York's Metropolitan Opera House, the first time a rock act had performed at the Met.
1971: Montreal's Les Grands Ballets Canadiens stage a dance "interpretation".
1972: The London Symphony Orchestra (with guests Richard Harris, Rod Stewart, etc) record their version.
1972: An all-star LSO performance at the Rainbow Theatre in London features David Essex, Peter Sellers, Bill Oddie, Jon Pertwee and Elkie Brooks.
1975: Audiences sit through Ken Russell's incomprehensible film, with Elton John, Tina Turner et al.
1979: Tommy returns to the West End stage - without
April 1993: Rewritten, with extra songs and a stronger storyline, The Who's Tommy opens on Broadway, where it picks up five Tony awards.