Lance Hammer, an American film-maker of great rigour, has made just two features in nearly two decades. His first, Ballast, set in the Mississippi Delta, won the directing award at Sundance in 2008 and was widely regarded as one of the best American films of that year. It even made the late critic Roger Ebert cry.

Hammer’s second, equally powerful feature is Queen at Sea, starring Juliette Binoche and Tom Courtenay, which has already earned worthy comparisons with such powerful depictions of later life and dementia as Gaspar Noé’s Vortex and Michael Haneke’s Amour.

It has been worth the lengthy wait. But where has he been?

“I wanted to make something that in the creative process itself gives me meaning,” Hammer says. “In America you can’t make money making independent films, so the creative process is my payment.

“At a certain point I just felt, like, if I can’t make the kinds of films I want to make, then I’d rather do something else. I had to make a living doing regular stuff. I was an architect by training. So I did a little architecture. I worked in investment and lived a regular life, but I kept writing things all this time.”

When the idea for Queen at Sea first came to him, Hammer was reluctant to tackle the thorny subject matter.

“I thought, I don’t want to make a story about the elderly,” he says. “I don’t want to make a film about dementia. I don’t want to do this. But it just kept coming out easily. I really wanted to finish the screenplay at that point, just for my own exercise.

“This is really challenging subject matter, and if you’re going to tell it, you have to tell it truthfully. You can’t make it a redemptive, sentimental story, because that’s not true. That’s not how it works.”

Hammer’s uncompromising film opens with a desperately sad dilemma. Amanda (played by Binoche) walks in on her mother, Leslie (Anna Calder-Marshall), having sex with her stepfather, Martin (Courtenay). Amanda immediately scolds Martin: Leslie has advanced dementia, and Amanda believes her mother is no longer capable of giving consent to her doting husband and carer.

She then calls the police – this is clearly an ongoing dispute – which bring Martin and the family into a bruising series of encounters with the law, medicine and social services.

“It’s not my own story or my family story,” Hammer says. “But my father is 88 years old, and there’s a lot of dementia around in his friend group. I’m close to my father, so I’ve been hearing and witnessing what’s happened to some of his friends.

“I think my radar started going up somehow. Then I came across a story that dealt with the sexual-consent component with dementia. And that was really fascinating to me, because the moral questions around it are different than for the cognitively intact. You have to rethink the rules. There are no easy answers.”

The film-maker has been a huge fan of Binoche, his first choice for the lead role, since Three Colours Blue.

“You feel something watching Juliette,” he says. “There’s this magnetism, this aura, this powerful life force. The pathos in her is amazing. She’s technically a master. She was the first person on the project, and we were on it together alone for a long time.

“She was actually a producer – a development producer – although she didn’t want to take that credit. And together we scraped it off the ground several times when it fell apart.

I really believe in this workshopping process where you start with the foundation of something … You find actors that really want to work this way. And you cast actors that have the unique ability to bring their own souls and emotionality to a character

“But the thing I most admire about her is her commitment to artistic integrity. She thrives on the creative process. She’s addicted to it. Nothing can compromise it. And I feel very much that way myself.”

Courtenay and Calder-Marshall, who were jointly awarded the Silver Bear for best supporting performance at this year’s Berlin film festival, have been friends since playing Hamlet and Ophelia in a celebrated 1968 stage production.

Courtenay, famed for his defining performances in the British new-wave films The Loneliness of the Long Distance Runner and Billy Liar, has twice been nominated for an Academy Award: for Doctor Zhivago, from 1965, and for The Dresser, from 1983.

Calder-Marshall, who also played a famous Cordelia opposite Laurence Olivier’s King Lear in a television production, has remained an extremely familiar face from series such as Morse, Poirot and, opposite her son Tom Burke, Strike.

That longevity and experience proved useful in the making of Queen at Sea.

“They’re incredibly powerful machines of artistic talent,” Hammer says. “I loved Tom as an actor, but then with Tom came all this other stuff, including his history. And with that history came this incredible real-life relationship, which translates to the screen between Tom and Annie.

Willem Dafoe: ‘I wasn’t a man about town. I was a kid trying to make theatre in a tiny company’

“It feels like absolute luck. I did not plan it. I had no idea it existed. I couldn’t write it. They brought it, and we captured it. The same with Juliette. She just brings a whole tour de force of emotional energy. You have to get out of its way and let it happen.”

With Ballast, Hammer was less interested in dialogue or conventional storytelling than in the texture of life in the Mississippi Delta: its winter light, its soundscape and its unhurried rhythms. He recruited nonprofessional actors, giving them situations and loose instructions rather than conventional dialogue, allowing their responses to shape the film.

Queen at Sea was similarly fashioned from a skeletal story. Some scenes, notably Martin’s account of meeting his wife at a Tchaikovsky recital, were improvised by Courtenay, recorded by Hammer and subsequently written into the script.

“I really believe in this workshopping process where you start with the foundation of something,” the director says. “You’re not starting from scratch. You find actors that really want to work this way. And you cast actors that have the unique ability to bring their own souls and emotionality to a character.

“Not everybody can do that. I’m asking for the real person. I want the real Juliette Binoche. I want actors – usually Juliette – who say, ‘I would do this differently,’ or, ‘I wouldn’t react that way.’ Tom really invented a lot of his language to make the film not only British but his own way of seeing the world. That’s where the real truth comes from.”

A parallel plot involves puppy love. While Amanda, Martin and the authorities argue over care arrangements, Amanda’s teenage daughter, Sara (played by the Bridgerton star Florence Hunt), roams around north London and finds romance with a local boy.

This narrative counterpoint required an intimacy co-ordinator.

“We wanted to make sure that everybody was comfortable,” he says. “The first step is to show that, as in production, you’re not trying to create any exploitative moments.

“I never worked with an intimacy co-ordinator before, so I was worried that I would be told to do things with the script or the story. But in fact Jenefer Odell really protected our story. She was a great creative collaborator. We all sat down and figured out what blocking felt comfortable.”

Although Hammer spent a month rehearsing the material, he never discussed the film’s central moral quandary with his cast.

“I didn’t want to put my finger on the scales,” he says. “When I watched Tom and Juliette’s characters at loggerheads, I was curious to see if I could decipher what their actual positions were morally. But everybody kind of kept it close to their chest. I still don’t really know what they think.”

Queen at Sea is in cinemas from Friday, September 18th