Sir Ian McKellen has a confession to make. His most famous line, in a celebrated film and TV career that has lasted 60 years – Gandalf’s “You shall not pass!” in the Lord of the Rings trilogy – was a mistake.

“Don’t tell anybody, but I got it wrong,” one of Britain’s greatest actors says over Zoom from his London home. “The actual line in the book is ‘You cannot pass’. Quite whether I did that with the connivance of Peter Jackson, the director, I don’t know, but Tolkien it ain’t.”

McKellen, now 85, is waiting to hear whether Jackson wants him to be involved in two planned new Middle Earth instalments that will start with Andy Serkis directing and starring in The Lord of the Rings: The Hunt for Gollum. “He’s producing some films about some of the characters who we know from the blockbusting movies, and if Gandalf turns out to be one of them I think I shall be invited to return Down Under, and I would do that with great pleasure,” he says.

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Even away from Middle Earth, McKellen is still getting the kind of meaty roles that actors love.

In Anand Tucker’s thriller The Critic he plays veteran theatre critic Jimmy Erskine, who dishes out savage reviews for The London Chronicle in the 1930s. But when the paper’s longtime proprietor dies and ownership passes to his son, Viscount Brooke (Mark Strong), Erskine is on shaky ground.

His expenses are extravagant, his reviews of actress Nina Land (Gemma Arterton), who is Brooke’s secret crush, are poisonous, and he has a run-in with the police over his gay assignations in a park.

“If a script comes my way and the scriptwriter – Patrick Marber, in this case – tells you it’s the best part he’s ever written for an actor, well, you prick up your ears and get ready to say, ‘when do we start filming?’,” McKellen says with that famously sonorous voice that has lent weight to such characters as King Lear, Macbeth, Hamlet, Sherlock Holmes, Magneto in the X-Men films, director James Whale and Adolf Hitler.

“But the first question I always ask myself when I read a script is, ‘is this a movie that I would want to see?’ And it reminded me of many melodramatic stories from the past in the movies, from Casablanca to All About Eve, really: goings-on among show-business people.”

McKellen thought a story about a character as malicious and manipulative as Erskine was “both credible and playable” at a time when a newspaper critic could make or break a show.

“I found that to my cost, working on Broadway in New York when The New York Times seemed to be, in the old days, the arbiter of taste,” he says. “That was a play I’d done in the UK for a year successfully on the West End. It was a Russian play called The Promise and it showed everyday Russian people having the same motives, desires and fears as the rest of us.

“But this was right in the middle of the Cold War, when the Americans and the Russians were at total loggerheads and the Americans weren’t ready for the idea that the Russians were ordinary human beings. So, instead of a year on Broadway, it was just under three weeks and we were back home.”

So, is playing Erskine his revenge on critics?

“Not at all,” McKellen says. “I was at school with the son of the local critic who wrote for The Bolton Evening News in the north of England where I lived,” he says. “I knew that man as an amateur actor in a troupe I worked with as a kid – so not frightening at all.

“And I’d even had a few reviews in the local paper by the time I went to university. It was there I had a great review in a newspaper for an underground production where the actors were all anonymous. This review said he wished my name had been in the program because it was a name that might be remembered.

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“Well, that was very alluring stuff for an 18-year-old amateur actor who had never really thought seriously about becoming professional.”

Knowing such budding actors at the time as Derek Jacobi, David Frost and Peter Cook, McKellen says he wondered, “Wow, perhaps I am good enough to give it a go”.

“So it was a direct result of reading a review that I decided to become a professional actor, and I’ve always been rather favourable to them ever since.”

Has McKellen come across any critics as venomous as Erskine?

“There was a man whose name I’ve deliberately forgotten who wrote in New York – not for The New York Times – and his stock-in-trade was simply to be rude about people’s appearance, particularly female actors,” he says. “He got his comeuppance one day when a woman went to the same restaurant he was in and poured his plate of spaghetti all over his head.”

McKellen, who was a rarity among actors when he came out as gay in the late 1980s, says Erskine’s sexuality resonated with him.

“I was born in the 1930s, just,” he says. “The predicament that Jimmy Erskine had was what we all had: born gay but because of society’s rules and misunderstanding, born a criminal … That all seemed to ring true to me and made me realise what I did already know, that to be alive [now] as a gay man, at least in my country, and yours, of course, is much preferable to what it was like in the 1930s.”

Despite toppling from a London stage while playing Falstaff in June – he quickly recovered – McKellen has no intention of retiring.

“I haven’t brought up a family,” he says. “So the idea of retirement, playing with the grandkids, it’s not an option.

“But I find work so fulfilling and so enjoyable. It gets me out of the house, I get to join in with a group of people who, if things go well, rapidly become a family. That would apply whether you’re doing a long run of a play or on location with a movie. That’s been my way of life for the last 60 years.

“So as long as I don’t have too many accidents, as long as my knees won’t give way, as long as the brain holds up, I shall go on working.”

Given Erskine suffers from generational change in the newspaper business, what does McKellen think of Rupert Murdoch’s court battle with three of his children over control of his media empire after he dies?

“I’ve played King Lear and I knew when Rupert Murdoch said he was going to retire and hand everything over to his children that it would work out badly,” he says. “Read King Lear, Rupert. You can’t give away everything and hold on to power. It’s one or t’other.”

The Critic opens in cinemas on October 3.

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