For some American moviegoers, “The Lady in the Van’’ will be an introduction to Alan Bennett, one of Britain’s most popular and prolific playwrights, and to Mary Shepherd, the irascible elderly woman who for 15 years lived in a van parked in Bennett’s driveway in Camden, north London, starting in the early 1970s.

But it’s a familiar story to Londoners, particularly Bennett’s neighbors on the tony street known as Gloucester Crescent. Mary Shepherd, played by Maggie Smith, who also starred in the London stage version of Bennett’s autobiographical play, was at best tolerated and at worst reviled. Fleeing a fascinating but troubled past and a possible crime, Shepherd hounded Bennett to let her park her beat-up van in his driveway. He relented, much to the chagrin of other residents who, as the years wore on, grew used to the sight — and the stench — of Shepherd and the van she called home.

Director Nicholas Hytner, who directed Smith in the original stage production, says Shepherd deliberately chose Gloucester Crescent as her landing spot. “It was a little colony of writers, artists, directors, and journalists, and they all bought [homes] when Alan bought in the ’60s. It was a haven of liberal intellectuals,’’ he said in an interview at the Toronto International Film Festival. Hytner shot “The Lady in the Van,’’ whose screenplay is also by Bennett, in and around Bennett’s old house.

“Stephen Frears’s kids lived on Gloucester Crescent with his ex-wife, and he spent a lot of time there. The house was directly opposite Bennett’s, and his kids Will and Sam were among those who’d [playfully] torment Miss Shepherd,’’ said Hytner of his fellow director. “We were shooting one day and [Frears] came over and said, ‘What’s going on here?’ I said, ‘We’re shooting a film about Miss Shepherd,’ and he said, ‘You won’t find me paying money to see that! I knew the real thing. You might as well make a film about Goebbels!’ And off he went,’’ Hytner said, laughing.

But with “Downton Abbey’’ in its final season, plenty of people stateside will want to see Smith — the two-time Academy Award winner who has garnered recent acclaim and new legions of fans as the acerbic Violet Crawley, Dowager Countess of Grantham — as the eccentric Shepherd, a role that earned Smith a best actress nomination at the 2000 Olivier Awards. She played her again in a 2009 BBC radio adaptation.

“We talked about doing it as film in 1999 or 2000, but none of us can remember why we didn’t do it, because Maggie was always up for it,’’ Hytner said. “With Maggie in it, we could always get it financed.’’ Two years ago, Hytner directed Alex Jennings as Alan Bennett in a National Theater production of two plays, “Hymn’’ and “Cocktail Sticks,’’ both from another Bennett memoir, “Untold Stories.’’ “He was so good as Alan that we thought, Let’s make this film at last,’’ Hytner recalled.

Despite being 80 years old, Smith “hauled herself at it,’’ Hytner said. “There’s something about the bloody-minded independence of Miss Shepherd. Even though she’s at the bottom of the heap, broken and derelict, her determination to bend the world according to her will is what makes it so watchable. There is nothing lovable about that character; she’s a monster, but she’s so fiercely determined, and nobody is going to tell her what to do, that you go with her. She never once says ‘Thank you,’ she never acknowledges that anybody is doing her a favor.’’

In the film, Jennings, as Bennett, has an ongoing dialogue with himself (the writer versus the ordinary person) about why he’s let Shepherd invade his quiet world. Jennings says it was a question that he also wanted to answer when he, Bennett, Hytner, and Smith met to rehearse before shooting. “Alan claims it was not about kindness, though he is a kind and good man,’’ says Jennings. “[Shepherd] had a will of her own and he just wanted to get back to work.’’ The film comically speculates on the complex reasons for Bennett’s behavior. Is he guilty over neglecting his own mother? Or are they simply, two misfits who grow to need one another?

Jennings’s relationships with Hytner, Bennett, and Smith go back decades. He first starred with Smith in a 1993 London production of “The Importance of Being Earnest,’’ directed by Hytner.

“Maggie was happy to have me on board, and I’m grateful for that,’’ he said. “She is inspiring and demanding. She knows her stuff and she trusts me. The fact that we have a history helped a lot. I’d never had a lead role like this, and it gave me confidence to have [all] their blessings.’’

Jennings is no stranger to playing real-life characters — he was Prince Charles in Frears’s 2006 movie “The Queen,’’ with Helen Mirren — but the mix of reality and art in “The Lady in the Van’’ was sometimes downright spooky.

“To shoot the film in that house, sitting at Alan’s work desk and looking out the window where the van was parked, was a visceral experience,’’ he says. “[Shepherd] was just outside his front door and a big presence in his life.’’

Loren King can be reached at loren.king@comcast.net.