‘Would you like a beer?” asks Alan Alda, tall and elegant in black raincoat, grey jacket and blue jeans as he walks through his offices near the Lincoln Center in New York. The actor, director and science communicator warmly greets Einstein, a female half bernese mountain dog, half border collie. “Smartest dog I ever met,” Alda’s assistant later observes.

The urbane 83-year-old star of M*A*S*H, The West Wing and The Aviator settles in a glass-walled meeting room, acknowledges that we are here to talk about his new film, Marriage Story, but says he is happy to talk about anything. Over the next hour, he will discuss God, mortality, his mother, podcasting, science, Woody Allen and how he is, so far, unbowed by Parkinson’s disease.

Alda does not appear in the first hour of Marriage Story, probably the most wrenching film about divorcing parents since Kramer vs Kramer. But his role, as a world-weary lawyer with past divorces of his own, is exquisitely rounded. He says he is “more and more impressed” by how the director, Noah Baumbach, has “done the unimaginable and that’s to make a love story out of a divorce story”.

Alan Alda with Adam Driver in Marriage Story. Photograph: Wilson Webb

Partly set in New York, the film’s improvisational quality and wry study of marital angst bring to mind Woody Allen at a moment when the Annie Hall director is persona non grata. Allegations that he sexually abused his adopted daughter Dylan Farrow in 1993, when she was a child, have gained renewed attention following the birth of the #MeToo movement. Allen has always denied them and Farrow’s claims were dismissed by two investigations at the time.

In September, Scarlett Johansson, who has appeared in three of Allen’s films and also stars in Marriage Story, said she believed the director and would work with him again. Last week, Jeff Goldblum agreed and said he would consider doing so. Alda, who was born two months after Allen and has appeared in such films of Allen’s as Crimes and Misdemeanors and Manhattan Murder Mystery, appears to be in the same camp.

“I think he was on trial at least once and maybe twice, and at what point do you say innocent until proven guilty and, if you were acquitted, when do you accept it? I don’t have any personal indication that he’s guilty. I don’t like mob actions and it feels a little like a mob to me. I respect somebody’s decision not to work with somebody because they believe the person has not done good things. But I don’t like badmouthing people if they haven’t joined your opinion.”

I’m more comfortable with the uncertainty of reality than I am with wishes and hope Alan Alda

Alda is clear, however, in his support for #MeToo in the film industry and beyond. “I think it’s really important that women are giving voice to what’s been a plague for centuries,” he says. “When the #MeToo movement spreads to farms and offices, manufacturing floors, then we’ll see big numbers.

“It has the slight – in some cases not slight – disadvantage that it’s a movement. It’s not a set of laws where there’s an effort made for the punishment to fit the crime. Movements have that built into them. I hope it leads to more serious rules and systems of coping with it.”

Alda has seen both sides of marriage. His parents’ relationship was full of strife and ended in divorce. His father, Robert, was an actor and singer who created the role of Sky Masterson in the Broadway musical Guys and Dolls. His mother, Joan, had paranoid schizophrenia and tried to stab her husband when Alan was six.

“She was ill from the time I was born,” he says. “I was angry for a long time because I didn’t know why she behaved that way and I felt I didn’t have a real mother. When I look back, I realise that even with her illness, she loved me very, very much and expressed it in the only ways she could – in spite of the fact that she would tell me often that I was trying to kill her. So I got a lot of mixed messages but, before she died, I understood a lot better. And I went to a lot of trouble to try to make her last days as comfortable as I could.”

Alda and his wife, Arlene, a photographer and children’s author, have been married for 62 years and have three daughters and eight grandchildren. He says that, according to Arlene: “The secret to a long marriage is a short memory. I have a more pedestrian way of looking at it which is, when you’re in the middle of yelling at each other, which you will at some point, remember that you’re talking to the person you love more than anybody in the world and it might change the tone of the conversation.”

When Alda fell ill with an intestinal obstruction in Chile in 2003, he dictated a letter to Arlene before emergency surgery in case he never saw her again. “I had about two hours to live,” he says, matter of factly. “We just came across it recently and it was just as pedestrian as I imagined. But it was basically telling my wife I loved her. That’s the best kind of pedestrian.”

A few years ago, while asleep, Alda threw a pillow at Arlene; he had been dreaming that he was under attack and was throwing a sack of potatoes at the assailant. When he woke, he remembered a New York Times article that said acting out dreams can be an early sign of Parkinson’s disease. A few months later, he got the first visible symptom, by which time he was already on an exercise programme that now includes boxing, juggling, tennis and marching to music by John Philip Sousa.

Alda went public about his condition last year but, like Michael J Fox, whom he has interviewed on his Clear+Vivid podcast, has no intention of letting it end his career. His hand can be seen mildly shaking in Marriage Story. In the TV drama Ray Donovan he plays a psychiatrist who also has Parkinson’s – a little worse than his own. “So sometimes I have to fake it.”

Alda also has prosopagnosia, or face blindness, but is untroubled by self-pity, instead focusing on the precious present. “I love reality,” he says. “I’m more comfortable with the uncertainty of reality than I am with wishes and hope.

“I am sort of optimistic, so that represents hope for me but reality is even more connected to hope than that because what is, is” – he slaps the table – “so why waste time wishing it away? I have a natural tendency to deny it so I can get on with things but that’s not the same thing as saying: ‘Oh, why me?’ If not me, somebody else. Why anybody? Nature is uncertain and, to a great extent, destructive. For Christ’s sake, we all die.”

Nature is uncertain and, to a great extent, destructive. For Christ’s sake, we all die Alan Alda

Alda relinquished the religion in which he was raised long ago. “I’m not any kind of Catholic,” he says. “I haven’t come across any evidence for God.” Instead, he says he finds the beauty and wonder of the universe sublime enough. Asked whether he thinks death is the end, he riffs on the ubiquity of microbes and how they made the world inhabitable for all living things, before adding that it is extraordinary “that we’re gonna die and it’s so amazing that most of us live as if that’s not gonna happen”.

Ten years ago, he helped set up the Alda Center for Communicating Science at Stony Brook University in New York. Scientists feature heavily on his popular podcast, of which there have been about 75 editions so far – and which he tries to keep as politics-free as possible, in part because of the impact he feels it has had on lucidity. “I don’t talk about politics in public, but I am in favour of facts. I think science has been under attack for a while by a number of groups, some of which support the administration.”

In an age of fragmented media, Trump in the White House and a nation either appalled or thrilled by his views, unifiers seem in short supply. But a generation ago, in 1983, there was M*A*S*H, which drew more than 106m viewers for its finale. Alda, who played Hawkeye in that series – and a likable Republican candidate for president in The West Wing – hopes that his efforts on and off screen can encourage people to listen to each other and mend the frayed social fabric.

Alan Alda with Mike Farrell in M*A*S*H. Photograph: 20thCFox/Everett/Rex

Accepting a Screen Actors Guild lifetime achievement award this year, he said: “It may never have been more urgent to see the world through another person’s eyes than when a culture is divided so sharply. Actors can help, just a little, by just doing what we do.”

The speech moved people to tears. “I wanted to say something about how what we do might be regarded by other people as an exercise in narcissism,” says Alda. “Because we [actors] really perform an important function, which is to help people live other people’s lives vicariously and we do that through empathy, and we make a real contribution when we do it well.

“Marriage Story is really about what I spend a lot of my life trying to do, which is help improve communication. To be able to recognise another point of view with respect and not contempt? Boy, do we need that now.”

Marriage Story is in UK cinemas on 15 November and on Netflix from 6 December.