Unbeknown to his directors and co-stars, the 71-year-old, who plays a man with terminal cancer in the movie, had himself been diagnosed with the disease.
“I told them after it was finished. I didn’t want to be treated any differently from anyone else,” says Connolly, who announced last year that he had undergone successful surgery on his prostate, and was also being treated for Parkinson’s disease.
In one scene, his character Gordy, a grandfather preparing to celebrate his 75th birthday, must discuss his illness with his daughter-in-law Margaret (played by Amelia Bullmore).
“I had to say, ‘I’ve got cancer’. Amelia didn’t know I was telling the truth,” the comedian says with a laugh. “I’d never said that to anybody in my life before.”
Today, the father-of-five, who is married to former Not The Nine O’Clock News star Pamela Stephenson, is looking fit and healthy in green-rimmed glasses and a spotted denim jacket.
He hasn’t changed “a single thing” about his lifestyle since his diagnoses, but admits to being “kind of clumsy sometimes” as a result of the Parkinson’s, and is struggling to play the banjo as well as he used to – “which is a pain in the butt”.
It didn’t take the comedian – who also wears hearing aids – long to see the lighter side of his health problems, “because I’ve always been on the outside of it”.
“At one point, I went to see the doctor when they discovered I had the cancer thing. He was doing a drawing of my kidneys and prostate and how it works and the bladder and stuff, and he said, ‘You’re not going to die’,” Connolly recalls.
“I was absolutely shocked – it never crossed my mind that I might die. So I’ve never been deeply involved with it in that sense.
“I don’t go on the internet and look up Parkinson’s disease and see how I’m getting on. I just live my life and it’s clumsy and that’s it.”
Connolly will discuss his health struggles during his upcoming High Horse Scottish stand-up tour, as well as the recent referendum.
“I might separate the Yeses and the Nos,” jokes Connolly, who kept “very quiet indeed” in the run-up to voting.
“It sounds self-important, but I didn’t want to be influencing people. I don’t believe in that kind of behaviour,” he explains.
“I stayed purposefully away from it because I found myself on both sides. I thought I didn’t believe in independence, but I also didn’t believe in being stuck with [David] Cameron and those other jokes.
“I felt for Scottish people, because no matter who they vote for, they get what England votes for.”
So how does he feel about the end result? “I don’t know, I’m still on both sides,” he says. “No matter what the result was, half the country was going to be disappointed. It’s going to take a bit of getting over.”
For Connolly, spending time in his homeland for What We Did On Our Holiday (from the makers of BBC sitcom Outnumbered) was a delight – despite being “eaten alive by midges”.
“It was beautiful, up on the west coast. It was so superb. That liquid light you get is joyous, and the people are so jolly,” says the star, who indulged his love of fishing during filming breaks.
He also loved improvising with the three young actors playing Gordy’s grandchildren. In the film, the trio travel up from London to visit their grandad with their estranged parents (played by David Tennant and Rosamund Pike), who are determined to keep their marital problems a secret.
“I really enjoyed the company of those kids, they’re so intelligent,” says Connolly, himself a grandfather of two. “They were better than me at [improvising], much more honest. I was thinking about what would have the best effect, but they don’t care. They go straight in.”
Connolly feels like a man of 45 (“I used to feel 35! I must be getting old...”), and agrees that he doesn’t look 71 – a feat he attributes partly to some “revolting aloe vera and snail slime” moisturiser, which he picked up while filming upcoming comedy Wild Oats on the Canary Islands.
Quoting Gordy, who has lived an adventure-filled life, he adds: “I’m ‘more, more, more’, that’s my thing. I’m all for life... I think it’s lovely.”
What We Did On Our Holiday is in cinemas on Friday, September 26