Forget the room where it happens. The real action goes down in the room before it happens.

British photographer Simon Annand has been taking pictures of actors onstage in London for more than 30 years, but his latest book, “Time To Act,” focuses on those mysterious moments before the curtain goes up. A-listers such as Benedict Cumberbatch, Judi Dench and Cate Blanchett let Annand into their private backstage domains to witness their pre-show rituals.

“They do trust me,” Annand told The Post. “The very famous ones have been photographed many times and they can second guess the motives of the photographer.

He even photographed an early-career Phoebe Waller-Bridge, who’s on the book’s cover, in 2009. “It was probably three or four years before she had even conceived of, let alone written ‘Fleabag,’” he said. “It was evident to me that she was far too bright to hang around waiting to be hired like a normal actor.”

It’s that kind of drive that inspires him.

“For me, it is literally to show the discipline, skill and focus required of performing live repeatedly eight times a week. And to show that the actors who choose to take on that challenge are not just vacant celebrities,” Anand added. “They really do want the challenge.”

Benedict Cumberbatch

On the TV show “Sherlock,” Cumberbatch’s brilliant character’s mind moves a mile a minute. In the makeup chair, however, as he was painstakingly turned into a monster for the National Theatre’s “Frankenstein,” the actor was a bit foggy.

“He had just come back from New York on a plane and caught a cold, I think,” recalled Annand of their 2011 encounter, which happened on a Tuesday. “By Thursday, he had to learn how to ride a horse for the film of ‘War Horse.’”

The actor, now 44, also had a complex routine to get ready for the play, in which he and Jonny Lee Miller alternated the roles of Victor Frankenstein and his Creature every night.

“He had to go through an hour or two of this makeup. So he needed a lot of physical and mental concentration to get it together,” Annand said.

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Daniel Kaluuya

Kaluuya skyrocketed to fame with 2017’s “Get Out.” But before he hit it big in Hollywood, he was a scrappy stage actor. Playing a boxer in 2010’s “Sucker Punch” at the Royal Court Theatre earned the actor, now 31, an Evening Standard Award for best newcomer.

But it took extreme cardio to get there.

“He famously lost 42 pounds in weight during the three months that he was training to play the part,” said Annand, who was present for the actor’s grueling hourlong warmup.

“It’s not easy to do skipping [rope]!” he said. “And he was doing it for a good 10 minutes at that rate. He was set up to launch himself into a decade of work.”

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Cate Blanchett

Although Annand plies his trade mainly in Britain, he went backstage in New York a few times with his camera, including once to see Blanchett make her Broadway debut in 2017’s “The Present.” His kinship with the Oscar winner, 51, who was on the cover of his first book and wrote the foreward to this one, kicked off in 1999 with some hesitancy.

“The first time we met, she maybe didn’t necessarily want a photographer in her dressing room,” Annand recalled. “But she didn’t chuck me out! She could easily have said ‘I don’t feel like it today,’ and I’m not sure that day had been a particularly good one for her. I really admire that.”

Three years ago on Broadway was an easier — and more celebratory — encounter, even if Blanchett was smeared in blood for one of the drama’s scenes. “That evening, one of her children had a birthday,” he said.

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Kit Harington

When Harington starred in “True West” in 2018, he was one of the most famous ­actors in the world thanks to “Game of Thrones.” Returning to the West End, where he had appeared in “War Horse” a decade earlier, let him briefly escape the media spotlight for less-blinding stage lights.

“He told me he just got married to [”GoT” co-star] Rose Leslie,” Annand said. “So there was a lot going on in his life at that point.”

But Harington, now 33, had a serenity Annand didn’t remember from when the actor was 20. “He was concentrating more than I have seen him do in the past because of the weight of the responsibility,” the photographer said of Harington’s commitment to the Sam Shepard play.

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Judi Dench

“We’ve known each other since about 1984,” Annand said of Dench, 85, who starred in “Peter and Alice” in 2013. “On this occasion we just had shared a conversation about her husband, [actor] Michael Williams, who died in 2001.

“They were extremely close, famously close. He would give her 12 red roses every Friday,” Annand said. “The gesture of her arm is perhaps as if he was there on the sofa beside her.”