So just how good is The Simpsons Movie going to be? It may be only three weeks until its worldwide release on July 27 but the comedian and actor Harry Shearer – the man who lends his voice to yellow legends such as Mr Burns, Ned Flanders, Otto and Reverend Lovejoy – is as keen to see the finished result as the rest of us. Not that it is quite finished yet, he says.

He’s in London to perform with Spinal Tap, as the bass player in the spoof heavy rock band that gave him his first swipe card to comedy immortality, at the Live Earth concert at Wembley on Saturday. When he returns to Los Angeles next Thursday he has one last recording session to go. Then, finally, he hopes to see the film for which he’s spent the past year doing weekly recording sessions.

Is the film about environmental catastrophe? Does Bart skateboard through Springfield in the buff? Will Homer imperil and then rescue the planet? “I really don’t know I can talk about that,” he says with an apologetic frown, his eyes searching the London skyline for Simpsons snipers.

What he can own up to is the outlandish secrecy that has permeated the production. The cast were never sent scripts, but had to read each scene when they turned up to the recording session. As soon as each scene was finished, the scripts were promptly shredded.

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“If the security around the Los Alamos nuclear lab had been as good as the security around this movie I’d sleep a lot better at night,” he says.

The film’s 11 writers, including the show’s creator Matt Groening, have kept buffing up every scene and virtually every line throughout the past year. “I’ve done at least a dozen versions of one major scene,” he says, “and as late as June 12 I was doing a recording session, to picture, where they had two entirely different sets of lines for me to record.”

Shearer, 63, appears to half enjoy the preposterousness of all this, half resent the lack of trust it implies. In his 18 years in Springfield he’s had a few run-ins with the producers. He’s taken issue with scripts that he felt messed with his characters’ integrity, particularly one that completely rewrote the history of his character Principal Skinner. “I kind of rose on my hind legs on that one. If you build a bond with an audience you don’t negate it for no reason.”

Then there was the time in August 2004 when he was reported as saying in an interview that the show was in decline. The fur flew for a while. Did his outspokenness make the producers wary of him? “Well it wasn’t an interview,” he corrects. “It was a private e-mail that got leaked and was not meant for public consumption. But I think they’ve been wary of me long before that, possibly because I do write this sort of stuff for a living too.”

Although Shearer, along with the rest of the cast, makes millions of dollars a year from The Simpsons, something still rankles about the way that he feels his contribution is viewed. Do you ever get consulted on scripts, I ask? “Consulted or insulted?” he mishears.

Yet Groening is a friend, and Shearer would hate it if his complaints came across as ungracious.

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