Even if you don’t know Harry Shearer’s face – though fans of Spinal Tap will certainly be familiar with one of the great American satirists and comedy actors of the last five decades – you will have heard his voice. In The Simpsons, he plays Mr Burns, Waylon Smithers, Ned Flanders, Principal Skinner and more. “Mr Burns,” he says immediately when I ask his favourite character. “Pure evil is always best. When you play it, it’s a gift.”

While Shearer has been critical of the long-running animated sitcom’s quality over the last 20 years – “up and down” is how he puts it today – its 90s peak has aged remarkably well. “I don’t know. I haven’t seen it,” says Shearer. It’s on the TV all the time? “That’s what I hear! But it’s good to hear you say that.”

Sporadic press stories that he is fed up being on The Simpsons are, he insists, not true. “No, I still enjoy playing all these characters. It was the reason I did this instead of some other television show where you’re pretty much limited to one character. I liked the idea of the variety of characters, and I still like that.”

That said, he says the early days were more fun, back when episodes were recorded with all voice actors together in the same room. “It was one of the reasons I wanted to do it in the first place. Because normal animation is always done one part at a time, so you’re never hearing what the other actors are saying. It was appealing to be doing what I regard as real acting, which is hearing the other performer and taking that into account in your performance.”

There is certainly real acting involved in Here Comes J Edgar! Co-written by Shearer and Seinfeld writer Tom Leopold, the musical comedy tells the story of former FBI director J Edgar Hoover, one of the most infamous and controversial figures in American political history. It was performed as a one-off radio play in 1994 starring Kelsey Grammer as Hoover and John Goodman as his deputy Clyde Tolson. But it never made the stage until now.

‘Playing pure evil’ is a gift, says Shearer of voicing Mr Burns in The Simpsons (Photo: Disney)

On 10 July, Here Comes J Edgar! opens at the King’s Head Theatre in north London. It stars former Mad Men actor Bryan Batt and features the original music from Barbra Streisand composer Peter Maltz.

Hoover led the FBI for nearly 50 years from its inception in 1924, building and establishing the shape of US law enforcement, pioneering scientific advances such as fingerprinting. But he stood accused of egregious and illegal abuses of power and authoritarian campaigns against those he viewed as enemies of America: communists, immigrants, homosexuals and the black community, particularly during the Civil Rights Movement.

Hoover’s tactics included press collusion, blackmail and espionage; he would round up minorities into camps and deport them. “That sounds familiar, doesn’t it?” Shearer says. “Those tactics must be good. They’ve lasted this long.”

After his death in office in 1972, details slowly emerged of Hoover’s hypocritical double life: his secret decades-long relationship with Tolson, and his alleged penchant for cross-dressing at parties as his drag alter ego, Mary. “Hoover was a homophobic homosexual,” Shearer says. “His mode of operation was to collect as much information as possible.” He would wield that knowledge to make himself “unremovable” – he had files on everyone. “He had the biggest secret of all to protect, and the only way he could be sure of doing that was by accumulating power against every possible source of disruption of that.”

This all made alluring source material for Shearer and Leopold. “This guy built this legend that was so powerful that presidents couldn’t resist it. And so we thought it was a balloon ripe for puncturing.” It’s why they opted for a comedy musical. “The story of Hoover is too grim unless you’re having fun with it. It’s not going easy on him. It just means you had to make it even funnier.”

Shearer has created a new musical about former FBI director J Edgar Hoover with Tom Leopold, left (Photo: Charlie Flint)

But why has it taken so long for a full production? “For the executive class in showbusiness there are two words,” Shearer says, “and the easier one to say is no.”

I meet Shearer during rehearsals at a dance academy above a south London school. Now 82, he is small and slight, subtly charming and wryly amusing. He’s something of an Anglophile, owning a house in west London he shares with his wife, the Welsh singer Judith Owens.

He is also a Saturday Night Live alumni across two stints in the early 80s with the likes of Bill Murray and Martin Short. What does he think of its UK reboot? “Oh, I think Britain really needed somebody to come over here and teach them what sketch comedy is,” he deadpans. “But you’re talking to somebody who was on that show two different seasons and despised it. I don’t know why anybody does it anywhere. I wouldn’t do it on a far Pacific island.”

Shearer says the production of Here Comes J Edgar! began to become a reality last August. He was in the UK with Spinal Tap to perform a filmed concert at Stonehenge, and promote the long-awaited sequel to 1984’s This is Spinal Tap. It came out last September, but Spinal Tap II: The End Continues received a relatively muted critical response, even with a host of guest stars including Paul McCartney (“he’s improvising comedically – he’s great”) and Elton John. Shearer had long Covid during filming. “I was sick, so I wasn’t enjoying it much. I enjoyed going home at night. The crew were great and there were highlights. But I would have liked to have been doing it six months later.”

It was a lot to live up to after 41 years: the original was groundbreaking and adored, inventing the mockumentary style and so accurately skewering the ridiculousness of life in a rock band it has become its own short hand for things going wrong. “You’ve never seen more bewildered looks on your faces of any executive,” he says of the 20-minute demo they originally pitched to Hollywood producers. “‘What was that?’ Well, it would be a feature film. ‘Why would we do that?!’”

After a recent sequel, Shearer says Spinal Tap is finished (Photo: Studiocanal)

But for all its sleeper success, it didn’t make them any money. In 2016, Shearer led a lawsuit against Universal over lack of payments, eventually settled in 2019. So in another example of Spinal Tap’s pitch perfect critique of the music industry, like most old rockers who reform, Shearer admits the cash was a significant factor. “For years we thought, ‘We’re not going to touch that; we got lucky,’ but circumstances allowed us to say, ‘We didn’t make Jack shit off that first film monetarily, so we should try to get something going that would be more monetarily successful this time.’ It wasn’t all about greed, but it was that which opened the door to something that we had closed the door to previously.”

And was it worth it monetarily? He smiles and thinks. “What do I say about that? The returns aren’t in yet. It’s like election night in California.”

So is Spinal Tap done? “Yeah,” he says, and can’t be drawn further. He’s unsure if the Stonehenge performance will ever see the light of day after the murder last year of the film’s producer and star, Rob Reiner.

And what about the future of The Simpsons, and the possibility it could just continue for years with AI voicing the characters? “They better not,” he says. Has he thought about that? “Oh, I’ve thought about it. I can’t say anymore right now, but I’ve thought about it. There are people who are already leading the way and saying, ‘I own my voice, I own my likeness,’ in preparation for that.” He’s not really buying into the AI hype. “It’s going to make typewriters extinct. That would be my prediction.”

Shearer’s immediate focus is on Here Comes J Edgar! He says the intervening years haven’t altered the way he views the play, or Hoover himself. “No, no, it’s a comedy about a bad guy,” he says. “And I think his legacy is that for a long time nobody in the FBI dared to behave this way until now. It was Mark Twain who said history doesn’t repeat itself, but it rhymes. Yeah, it’s rhyming right now.”

‘Here Comes J Edgar!’ runs at the King’s Head Theatre from 10 July