No one is immune to bad reviews.

Take it from Sir Ian McKellen, who bared all in a 1974 production of “King Lear” in Brooklyn. John Simon, a critic for New York magazine, was largely unimpressed.

“When I took my clothes off, he gave my penis a review,” McKellen says with a grin over Zoom. “He complimented the penis, but didn’t think my acting was all that remarkable.”

A half century later, the British stage and screen legend is turning the tables with “The Critic” (in theaters Friday), in which he plays an acid-tongued reviewer named Jimmy Erskine. Set in 1930s London, the fictional drama follows the prickly bond between Jimmy and theater actress Nina Land (Gemma Arterton), a constant target of his catty takedowns. She reluctantly agrees to help him save his job from newspaper executives, who are incensed by his vitriolic scrawls.

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McKellen, 85, has always had a healthy relationship to criticism. As a young man, he acted in nearly two dozen undergraduate productions while studying English literature at Cambridge University. He recalls one glowing write-up from the time, which singled him out as “a name to remember.”

“That day, I decided to become a professional actor, because perhaps I was good enough to give it a go,” McKellen says. “And I haven’t regretted it ever.”

Critics haven’t always been so kind. (He remembers doing “Hamlet” in 1971, when the Sunday Times remarked that the “best thing” about it was “the curtain call.”) But he’s learned to let those notices roll off his back.

“You try not to dwell on bad reviews,” McKellen says. “That doesn’t stop me from reading them, but I long ago stopped worrying about critics. I don’t take them to be any more reliable than, say, a friend who’s just seen a show they recommend.”

Ian McKellen considers 'The Lord of the Rings' to be his 'Casablanca'

Over his six-decade film and TV career, McKellen has portrayed powerful mutants (“X-Men”), grizzled detectives (“Mr. Holmes”) and sentient clocks (“Beauty and the Beast”). He’s received five Emmy nominations and won Golden Globe and Screen Actors Guild awards. Still, he's allergic to compliments.

“I think I've been overpraised,” McKellen says. “If you're playing the leading part in Shakespeare and you say the words intelligently, you probably don’t need to do much more than that.”

He earned his first Oscar nod in 1999 for the elegiac “Gods and Monsters,” playing gay “Frankenstein” director James Whale. McKellen, who is also gay, calls it the proudest role of his career.

“It’s a remarkable character study,” McKellen says. “When I look back at recordings of my early stage work, I'm not very impressed. It's a great relief to me that I've gotten better as an actor. And I have done some good work, which will merit being viewed after I’m dead.”

That includes “The Lord of the Rings” series, for which he earned his second Oscar nomination in 2002 playing the wise and selfless wizard Gandalf.

“It’s been the great, unexpected joy of my career to have been in a film that is already a classic – my ‘Casablanca,’ as it were,” McKellen says. “It means I can go into a gathering anywhere in the world and there will be somebody who wants to talk to me because they're fans of those movies.”

McKellen is delighted to have his “own catchphrase”: “You shall not pass,” which Gandalf declares in “The Fellowship of the Ring” while saving his comrades from the fearsome Balrog. He remembers shooting the iconic showdown with director Peter Jackson and a yellow tennis ball, which would later be replaced by digital effects.

“I said to Peter, ‘I’ve got the staff. I’ve got the sword. I’ve got the ball in my eyeline. What does the Balrog look like?’” McKellen recalls. “He said, ‘We have no idea yet – we’re going to create all that on a computer.’ So there was one take where I said to the tennis ball, ‘You! Shall not! Bounce!’

“There was a lot of that movie I didn’t quite understand while I was filming,” he adds. “I didn’t know how everything would fit together, but Peter knew, my goodness me.”

'The Critic' star says it's 'hard' to accept 'you're not immortal'

McKellen, who lives in London, has primarily worked in theater recently. (He’s particularly pleased with his latest stab at “Hamlet,” which was reimagined as a film.) Anand Tucker, who directed “The Critic,” is in awe of the actor’s prolificacy.

“I’ve been very inspired by how alive he is at 85,” Tucker says. “He is so hungry for life, and open to learning and doing and experiencing all the time. There’s a relentless joy that’s so refreshing.”

But in June, McKellen was sidelined when he fell off stage during a West End production of “Player Kings,” which left him with a chipped vertebrae and a fractured wrist. The accident forced him to withdraw from the run, which he feels “ashamed and guilty” about.

Inside, “I’m 12 years old, but I also feel I am 85,” McKellen says. “Human beings trip all day long, and they don't even notice it until they're in their 60s. But when they're my age, they do have to be careful because a trip can be a broken hip and you don't really want that.”

He says he’s feeling better now, although “I’ve decided to not work until next year, when I’ve got work for the whole year if I want it. It’s hard to come to terms with the fact that you’re not immortal – we all think we’re going to live forever, don’t we? But if my friends are not dead, they’re often limping and struggling with their health. So I’m very, very lucky.”

In his downtime, the actor loves to take in classical music, and recently enjoyed Barry Manilow and Elton John concerts. He’s eager to return to New York: The last Broadway show he saw was Bette Midler’s “Hello, Dolly!” in 2017 (“I was so close to the stage, I could feel the breeze from her skirt”). He also wants to indulge in another slice of Junior’s cheesecake (“The greatest thing to come out of Brooklyn – I can taste it now”).

Ultimately, “I hope I come back to Broadway once more before I stop,” McKellen says wistfully. “It’d be nice to come and say goodbye.”