Its main commercial attraction may be the novelty of seeing a heavily made-up Anthony Hopkins impersonate the Master of Suspense, but the heart of the new biopic “Hitchcock” isn’t Sir Alfred. It’s the director’s wife, Alma, little known to the public but an enormous influence on the auteur’s films.

Helen Mirren, who plays Alma, is no stranger to bringing historical figures to the big screen; among other outings, she won an Oscar for playing Elizabeth II in “The Queen.” One assumes it was something of a relief, this time, to play a woman whose voice and mannerisms weren’t already well known to viewers across the globe.

“Yes, absolutely,” she says, noting that the entire burden of “the impersonation side of it” fell upon her co-star this time.

“I do wish I looked more like Alma,” she admits, “because Alma was a birdlike person, and the visual imagery of this tiny little woman with this huge, big man – and she’s the only one who’s got influence over him – I just loved that idea.”

Beyond some still photographs, there was little for Mirren to base her performance on physically. Although Alma Reville teamed with Hitchcock near the beginning of his public career (she was a film editor when they met in the ’20s) and worked closely with him until his death in 1980, she stayed out of the spotlight her husband filled so distinctively.

“There’s no film” of her, Mirren complains. “Just one tiny bit on YouTube, of Hitchcock getting his (American Film Institute) award, and there’s Alma sitting next to him. I watched that over and over again, but they only cut to Alma” – she snaps her fingers – “for a nanosecond. It was so frustrating.”

Instead, the actress relied on books – especially one written by Patricia Hitchcock, the couple’s only child.

“Here’s the daughter of an incredibly famous film director, and she chooses to write her book about her mother,” Mirren says. “That speaks so much about what she felt her mother’s contribution was.”

Historians agree that Alma was an intimate collaborator. In “Hitchcock,” which focuses solely on the risky production of “Psycho,” we watch as she puts her mark on one of the most famous moments in movie history. Alfred is adamant that the shower scene should have no score accompanying it; only Alma can convince him that it will play better with music. Anyone who’s ever heard “Skreek! Skreek! Skreek!” while pulling back a shower curtain has her (and composer Bernard Herrmann) to blame.

Alma was only occasionally credited officially for her efforts, usually as a contributor to the screenplay. But as Mirren puts it: “She had her finger in so many pies. She could say: ‘No, that costume doesn’t work; it needs broader shoulders. That scene doesn’t work; you should cut it there.’ People say ‘Why didn’t she get a credit?’ Well, it’s hard to credit that. You know: ‘Adviser’? ‘Wife-adviser’?”

Asked what she thinks Alma’s career would have been like if she hadn’t met Hitchcock, Mirren notes how fluid production roles were in the early days of movies, when everything was being invented – “You could do props and do lights, you could do the script. You could do everything, and they did all do everything” – before deciding she would probably have settled down to become a great editor. “Indeed, traditionally, and to this day, that has been an area of filmmaking that women have always had a very strong position in.”

Mirren isn’t so confident speculating on one of the film’s more imaginative subplots: While working on a screenplay for a non-Hitchcock movie with Whitfield Cook (who had helped write “Strangers on a Train”), Alma and the writer seem dangerously close to having an affair.

The actress says she has read differing accounts of this friendship but wasn’t as concerned about whether it was, in fact, romantic as she was about what the work meant for Alma and Alfred’s partnership.

“I suspect she wanted to work in an independent creative way,” Mirren says, “and I can see that being something that might have driven Hitch to jealousy. Nothing to do with sex and love affairs; another kind of jealousy.

“The whole power of the two of them was the partnership, you know. They complemented each other so incredibly well and held each other up. I think together they were much stronger than they would have been apart, and when you take one side of the partnership away… . ”

If the movie’s singular title – it’s “Hitchcock,” not “The Hitchcocks” – plays down that partnership for the sake of drawing in viewers, perhaps this was showmanship that Alma would have endorsed. Mirren notes the extent to which the name “was a brand. He was such a great marketer, and he marketed himself as this” – she mimics the director’s plummy diction – “ ‘good e-ve-ning’ person. The shape and look and the suit. I think they both understood that was a good thing, and you didn’t want to mess with the brand.”

Mirren herself will soon return to playing a woman who had no choice but to live in the spotlight. Next year she’ll star in “The Audience,” a stage production in which Peter Morgan (writer of “The Queen“) imagines meetings between Queen Elizabeth II and the many prime ministers who have served during her reign. (The 67-year-old Mirren will play the monarch from age 25 through her 80s.)

She isn’t, however, likely to play that queen of white-bread domesticity, Martha Stewart, despite occasionally having spoken of her as a fascinating character in interviews. She turned down a Stewart biopic once, finding it “mean-spirited and wrong,” then wound up using her as inspiration for an unusual role: the fierce but refined former CIA operative Victoria in 2010’s “Red.”

“I said I could see Martha Stewart having a secret life as an assassin,” Mirren recalls. Fans who enjoyed the sight of a Martha-inspired killer wielding machine guns alongside Bruce Willis will soon get another opportunity to enjoy Mirren in that role. The actress is making a sequel to the shoot-’em-up comedy, which should hit theaters shortly after “The Audience” ends its London run.