“I remain pessimistic,” announces Woody Allen. “Nothing has changed.” As permanent as those black-framed glasses of his, the 79 year-old actor-writer-director of Annie Hall is still marvellously maudlin. We’re sitting in a darkened room in a hotel in Cannes, just to add to the gloom. “I always had a very dark view of life, a very dark view of people and I still do. If I could kill all the people I wanted to kill, I would be the only one left in the world.”

He is joking – hopefully – but this black cloud that hovers over the comedic Allen and much of his work seems fitting for his latest film, Irrational Man. Set in a leafy Rhode Island college, Joaquin Phoenix plays Abe Lucas, a philosophy tutor who drinks, womanises and becomes increasingly depressed. Until, after overhearing a chance conversation, this man of reason is inspired to murder a corrupt judge, an anonymous act of terror that revitalises him.

Returning Allen to Crimes and Misdemeanours territory, it also draws from the philosophers that he’s devoured since he was young (his first wife, Harlene, was a philosophy major). The question is age-old: to be or not to be. “Sometimes, you make a big mistake if you don’t take action,” he nods, “and other times, you make a big mistake if you do take action.” It’s the sort of conundrum philosophers, at least until the all-action existentialists, have wrestled with for centuries.

Unlike Abe, Allen’s never been a man of action. “I never got carried away, only in my room,” he says. “I’ve never had a puff of marijuana in my life, I’ve never done anything…I’m a lower-middle class, non-courageous, coward. I’m not a curious person. I’m not curious to travel – I could live my whole life within twenty blocks of my house if my wife would let me. I’m not curious to experiment with anything…I don’t know why.”

Still, given Allen lived through the Sixties, was he never tempted to inhale a puff or two of marijuana? “No, and I was in the thick of it, I was a cabaret comedian. We would do a concert and then afterwards everybody would go out and I was with jazz musicians and folk singers and no…I was just never curious. And people double my age at the time couldn’t wait to try it. They wanted to try LSD and they wanted to try cocaine and it was just nothing that interested me.”

Raised in Brooklyn, the son of Nettie, a book-keeper, and Martin, an engraver, Allen was more interested in sedate recreational activities – magic tricks and playing his clarinet (which he still does today, nearly every Monday, at New York’s Carlyle Café, with his jazz ensemble). By 15, he was writing jokes for newspapers, before embarking on a stand-up career that eventually led him to pen the 1965 comedy, What’s New, Pussycat? – the first of a staggering fifty films he’s either written, directed or starred in.

Of course, life in the public eye hasn’t been easy for Allen – notably his former relationship with actress Mia Farrow, who starred in a dozen of his movies. Together for twelve years, they adopted two children, Dylan and Moshe, and had one biological child, Satchel, but their relationship imploded when Farrow discovered he’d been having an affair with her adopted daughter Soon-Yi Previn (from an earlier partnership with conductor André Previn).

It led to a swirl of negative publicity that hasn’t really left Allen; in 2013, Dylan revived accusations that Allen abused her when she was 7, something Allen vehemently denied in a letter to the New York Times. For his part, he’s remained happily married to Soon-Yi since 1997 (they have two adopted daughters, Bechet and Manzie). “In the end it’s all luck,” he says, on what makes a good union. “It’s not easy to find. You really have to be lucky.”

He’s more at ease talking about Diane Keaton, arguably his greatest muse. They dated briefly in the Seventies, before making eight films together. “Before her, I only wrote for men. When I was living with her, I got to know her and see things the way she saw them. And I wrote Annie Hall for her.” The film won Allen two Oscars – he claimed two more for Hannah and her Sisters and his recent surprise hit Midnight In Paris; Keaton also took Best Actress.

They last worked together on 1993’s Manhattan Murder Mystery (and Allen’s film-a-year form has been patchy ever since). Do they not think about making one more movie together? “Sure, we talk about that,” he says. “I have a very good chemistry with Diane, and if I could think of something we could do…I don’t want to do a geriatric film! And I can’t play the leading romantic man anymore.”

Regular as clockwork, Allen is about to start shooting his next film with Jesse Eisenberg and Kristen Stewart. But this year, he’s added to his plate – writing six half-hour TV episodes from Amazon Prime. “It’s a nightmare!” he cries. “I thought it was going to be easy, but it’s not easy. I thought, ‘How bad can it be? You do six half-hours.’ That takes me no time. But I’m not good at it. I just hope I don’t disappoint Amazon.”

So prolific he still cranks out short stories if he has “a week or two off between projects”, surely this huge appetite for work must keep him young? “No,” he says, shaking his head. “Nothing keeps me young! The closest thing I can think of is emotional immaturity! That keeps me young. But nothing really keeps you young. You age, you deteriorate…you flame on for a little bit then you flame out. And that’s it.”

Irrational Man opens on September 11.