Ben Foster has been out on a boat with scallopers. The actor is studying fishermen as immersive preparation for his next film. He speaks to the Guardian via Zoom from a dock in Massachusetts. But the feed is audio only.

The thing about Zoom’s black screen is that the imagination tends to compensate and run riot. Is Foster sitting there drenched, clad in a sou’wester and wreathed in seaweed, king mackerel dangling from a hook? We will never know.

Born in Boston, he took up acting as teenager and made his mark, often brooding and mercurial, in films such as Alpha Dog, 3:10 To Yuma, The Messenger, Leave No Trace and Hell or High Water. Now 41, he has gained a reputation for diving deep into every role – all the way to the ocean floor.

In 2014 he took on Stanley Kowalski, famously played by method actor Marlon Brando on screen, opposite Gillian Anderson in A Streetcar Named Desire at London’s Young Vic. When the production crossed the Atlantic, the New York Times told how he had found inspiration for the character in apes.

“He examined pictures of Francis Bacon’s baboons,” the paper reported. “He screened YouTube videos of silverbacks encountering mirrors in the jungle. ‘Watching how they live in their bodies was very important to me,’ he said.”

When Foster portrayed a navy seal in the 2013 film Lone Survivor, leading cast members were put through a boot camp for a month. The Guardian reported that Foster was seen “scoffing handfuls of dirt”, prompting the headline: “Ben Foster: why I ate dirt for Lone Survivor.”

That is not quite how he remembers it now. “It wasn’t handfuls and so lore goes and myth goes and people like to over talk it,” he said. “The truth of the story is that I looked at my fellow actors who had just fallen down a mountain and they were laughing in between a take and I became starkly aware of their Hollywood teeth.

“I immediately became self-conscious. If you’ve just fallen down a mountain your teeth probably don’t look like that, so I just grabbed some dirt and put it in my mouth and the director caught me. It felt like you probably catch some in your mouth on the way down a mountain. It wasn’t that thoughtful, nor was it that impressive; it was actually unimpressive.

“It was just, my God, these people are filthy and covered in blood and this is a real story and we’re surrounded by navy seals who are helping guide us, but they’ve got sparkling American fucking teeth and that felt somehow apprehensible. My instinct was just to grab some dirt and put it in my mouth. It’s not like I’m hosing down dirt just to prove a point!”

With that he bursts out laughing.

Foster is evidently reluctant to embrace the term method actor, whatever that means. “Well, it’s a funny word and it’s a term that’s often misinterpreted and I probably don’t have a great interpretation or definition of it.”

He puts it another way. “Maybe this is a shitty thing to say and a cop out, but if you were to spend six months thinking about about making chairs – oh my God, I love wood, I love making chairs, the way the seat works – and you’re spending time with people who love chairs too and you just want to honour chairs, you’re going to think about chairs.

“Then when the film ends and you’re no longer making chairs, you’re going to dream about chairs. That’s what it is. It’s not fancy. It’s like, well, this is what we’re talking about and they were there, I wasn’t, so let’s think about it a little bit more rather than, ‘Oh, let’s go have a yoghurt together. Anyone want a smoothie?’”

Foster used this approach for the title role in The Survivor, directed by Oscar winner Barry Levinson, who cast him in his first film when he was 17. It is based on the true story of Harry Haft, a Polish Jew who, after being sent to Auschwitz, stayed alive by taking part in bare-knuckle boxing matches with fellow prisoners for the amusement of the Nazis.

Haft later settled in America and had a brief career as a light heavyweight, even taking on the mighty Rocky Marciano. Lyrically shot, the movie snaps back and forth between concentration camp and boxing ring. When it premiered at the Toronto film festival last year, Guardian critic Peter Bradshaw described it as “almost a case of Schindler’s List meets Raging Bull”.

It is also a love story – Haft is driven by the hope of reuniting with the woman he lost – and exploration of survivor’s guilt. After the war, Haft is seen facing recriminations from Jewish people for having done the Nazis’ bidding to save his own skin; he died in 2007. Did Foster, too, come to judge him?

“I suppose my job in my trade as an actor is to defend my character and it’s not to make excuses and it’s not necessarily to unravel the knot of ethical ambiguity. James Cagney has a fine quote about the work we do, which is plant your feet and tell the truth, and the truth is not all that simple or convenient.

“Which is rare, particularly as I’m sure you’ve experienced as an appreciator of cinema, as it seems to be in the last 15 years, people are less interested in the exploration of the human experience. They want to see the mighty action and the heroic failure and to my experience, at least in its limited view, it’s not clean, it’s not easily violable.”

“What Harry went through, there’s no easy answer here. He survived and he did it for love, and he wanted to see his loved one, the girl he loved. It’s interesting, I listened to hours of Holocaust testimonies, and there was a through line.

“Much of that through line was the hope of seeing someone that might be gone and the imagined hope that they might be reunited. I just want to see my son. I just want to see my wife. I hope that they’re there still, and the hope that they might was the thing that said, ‘I’m not going to lay down.’”

As part of his preparation this time, Foster visited the camps at Auschwitz-Birkenau. Asked what effect that experience had on him, he pauses. “I’m trying to find the words, I can recall, rather than an understanding, a sensorial experience.

“I needed to put my hands on the rails and listen. It sounded like the birds have still not returned. You see those piles of baby shoes, you see the piles of people’s belongings: their toothbrushes become these objects that defy thought.”

Haft suffered terrible weight loss in the camps; Foster shed 62lb in a remarkable physical transformation for the role. What dietary changes did he make to achieve it?

He replies laconically: “Less food.” Then he laughs.

“When a human body is denied sustenance, where does will come from? It’s that candle, that little bit of light that we keep inside, whatever that is, in the hope of seeing a loved one or let me get to the next holiday or I’m doing it for my friend or my friend died so I have to live for them.

“And then the consequences of surviving must come into play. It’s not clean. That’s what I appreciate so much about Barry. He not interested in the easy answers. He’s more interested in the behaviour and the curious idiosyncratic animal that the human animal is and celebrating it in all of its wounds.”

Foster took a similarly all-in approach to the hard-hitting boxing scenes and did not use a stunt double. He had not played a fighter before but both his grandfather and father boxed so he was able to learn from the latter. “The sweet science, as it’s called. There are only four moves in boxing so it’s a chess match.

“In the time that we’re representing the art form, Harry is not what you would call the most refined boxer, more of a survival brawler, and I took much joy. Even last night, I was going into a spiral, I returned to these 40s boxers. I haven’t been able to shake Harry.”

Actors such as Jared Leto and Forest Whitaker have been known to remain in character even after the cameras stop rolling. Foster, however, does not kiss his two young children goodnight while pretending to someone else.

“I’m not coming home and speaking to them as Harry. That would be lunacy. However, when at work, some people find it easy to turn off and turn on and I appreciate the strict distance and craft of those that can turn off and turn on and joke around. Nothing makes me happier than a laugh, but this particular film I’d be lying to say that it felt like I would be doing a disservice to the story I was telling if I wasn’t shoulders back and heads bowed.”

The notion of chameleon-like actors being able to transform themselves into any character has recently been tested by debates over “authentic” representation. The recent casting of Helen Mirren, who is not Jewish, as the former Israeli prime minister Golda Meir provoked criticism.

Did Foster – whose grandmother fled pogroms in Romania – feel it important that Haft be played by a Jewish actor? “This is very dangerous territory and I sense you’re poking,” he says. “What I’ll say is I feel so grateful that I was invited to explore this because it allowed me an opportunity to re-evaluate, investigate, ask questions and sense my own Romanian-Ukrainian roots on my father’s side.

“But let us not be so boxed in. I’m also Irish and let’s look at the struggles and the troubles of the Irish and let’s not be so contained there. Let’s look at Afghanistan and say this is about being a human being. It’s not a boxing film; it’s not a Jewish film, it’s about people. It’s about being a human being who is dealing with trauma.

“I suppose indirectly, not to duck your question but to duck your question, I’m glad for the job because it allowed a time in my life that’s been meaningful.and hopefully that time translates. But past that, I can’t speak to why people are cast for what they’re cast. Acting is representation and I’ll leave it at that.”

Indeed, there is no monopoly on suffering. Foster has just finished working on Apple TV+’s Emancipation, a film about the horrors of slavery in America’s deep south.

He explains: “It’s about escaping the camps that America created with the African American slave trade. The stark similarities that I learned, particularly after visiting Auschwitz. are staggering. We are students and, as students. it’s not uncommon to feel a burden of new information and it takes a moment to digest.”

The cast of Emancipation is led by Will Smith who, if nominated for another best actor Academy Award, will not be able to attend the ceremony. Smith was banned from the Oscars for 10 years after slapping comedian Chris Rock at this year’s event.

Asked for his reaction to the slap, Foster again chooses his words carefully after a long pause. “Do you that John Lennon song, ‘I’m just watching the wheels go round and round?’” he asks with a laugh. “That’s what I’m doing in Massachusetts outside a fishing port. It’s not that I don’t give a shit or have an opinion.

“I’m tired. I’m tired. My comment doesn’t mean fuck all. I’m an actor and that’s not to say that actors don’t have a place in society; plumbers do too. The boiler in my family’s house wasn’t working for a week. We didn’t have hot water and my wife and I were talking about, whoa, what do we do?

“Well, we heat up water, we have some fire, make a bath for the babies. That’s real stuff and whatever they dealt with, I wasn’t there. I can comment but I’m not going to add anything important to this dialogue, so I’d rather just say, ‘Hey, we’re still here. Ain’t that something?’”

Perhaps inhabiting a character deeply means that leaving them behind feels like a little death. Earlier, Foster mentioned that he hadn’t been able to shake Harry Haft. Is he lingering longer than most?

“It’s so awkward to talk about,” he admits. “Again to the chairs, right? Sometimes I find myself sitting in Harry and realise it’s time to stand back up and look at other things. But appreciate that I got to sit for a minute.”

• The Survivor premieres on HBO on 27 April with a UK date to be announced