This year is not the first year that Casey Affleck will (surely) be up for an acting Oscar. He was the underdog in 2008 when he received a supporting nomination for his slow-burning performance in The Assassination of Jesse James by the Coward Robert Ford.

This year Affleck is the hot favourite for Kenneth Lonergan's Manchester by the Sea and it's significant that he returned home to Boston for the film after previously making The Finest Hours in the nearby seaside neighbourhood of Gloucester.

"Manchester-by-the-Sea is one of the most beautiful places," he admits. "The light is incredible. Boston is my favourite place to work; it's where my kids like coming with me. It's like going home."

Sometimes taciturn and averse to self-promotion - which has not helped his career - Affleck is smaller in build than his perkier older brother Ben. His brooding nature is put to perfect use in Manchester by the Sea where he plays Lee Chandler, a reserved janitor dealing with a personal tragedy. Forced to return to his hometown to become guardian of his orphaned teenage nephew, he is left to deal with the tragedy and his ex-wife Randi (Michelle Williams).

We come to realise that Lee was a far happier person beforehand, so that Affleck was tasked with playing two very different people.

"Principally being that character meant showing the complexity of what Kenny had written," he says. "He's a magician with the words that makes people sound like an ordinary conversation but it doesn't stop there; it goes deep into the character. I learnt an awful lot about how to look for different ways of playing things, to think about a scene or to just remain open to things that seem counter-intuitive in the moment. Even if it was a sad scene it was so well written it somehow made you feel sad, but then you'd feel better."

A wry-humoured eccentric who has more in common with his best friend Joaquin Phoenix (his estranged wife Summer's brother) than with Ben or Matt Damon, Casey wasn't expecting to have such a strong reaction when he watched the completed film.

"I've never cried in a movie I've been in, just watching it, unless it was an utter disappointment," he chuckles. "But I did with this and I think that's a testament to the story and just how magically it worked."

The film's world premiere in Sundance last year had been a rare surprise moment. Everyone was left with lumps in their throats.

Damon, the film's producer, thinks the film is very special too. "I thought it was one of the most beautifully written stories I'd ever read. I never made it through without crying and I never made it through any cut of this movie without crying."

Interestingly, it's because of Damon's relationship with Lonergan that the film came about. In fact, the film's premise had originally come from Damon and his good friend John Krasinski.

"Seven years ago John and I went to Kenny with this idea as Casey and I had done one of Kenny's plays (This is our Youth) in London in 2002 and Kenny's our favourite writer," explains Damon.

"The idea was to make Manchester by the Sea my directing debut and John and I were going to be in it. But once I read a rough draft I begged Kenny, 'You should really direct this' and I was still going to be in it. Then in a really bizarre fit of generosity I gave the role to Casey!" he jokes.

Damon was in fact committed to a slew of blockbusters, The Martian, Jason Bourne and The Great Wall when Manchester by the Sea was ready to go.

"So many movies are so big and so loud now and have superheroes in them," he notes, perhaps sounding a little apologetic regarding the films he's been making of late. "So it's important to make one that's just about people.

"I actually told Kenny I wouldn't give this role up to anybody except Casey and he instantly took the role," Damon recalls. "I've been next to Casey when he's delivering dialogue and I know he's uniquely good at it."

So why are Ben and Matt more famous?

"This is just my opinion," notes Lonergan, "but I think some people eschew stardom a little bit and some people are comfortable embracing it. As a low-level celebrity myself I belong in the former category and I don't know what it is that allows someone to maintain their artistic integrity while still becoming an immense star. Most big stars in my opinion are not great actors and Matt is an exception. Casey's just a great actor. I think he gets a little shy when things go too well for him."

Who: Casey Affleck and Michelle Williams
What: Manchester by the Sea, directed by Kenneth Lonergan
When: In cinemas next Thursday