For the most part, actors have now sussed out the art of the Zoom interview. The trick is to do it in front of a completely blank background, so the interviewer cannot possibly glean any personal details from whatever you happen to be in front of.

James Cosmo does not subscribe to this notion. We speak in mid-November and, when his Zoom screen clicked into life, he was vaping up a storm before a massive, illuminated wooden Christmas tree. True, he has played Santa Claus many a time, most memorably for most in the Narnia films, but also annually at his son’s school’s Christmas grotto (he was so good his son didn’t recognise him). Yet who knew he was this excessively festive?

“Well, I actually do little Cameo things”, he explains “It’s like an online, would you call it a convention? Where you meet people? Anyway, I really enjoy doing it. I do a lot in America, and Thanksgiving is happening soon, so we thought we’d make it look a bit nicer to cheer people up.”

Cosmo has now been working in film and television since the mid-1960s, delivering a specific type of burly authority figure to hundreds of productions. Most will recognise him as Jeor Mormont, Game of Thrones’ original Night’s Watch commander, but that shouldn’t overshadow the rest of his work. The sheer number of classics he has appeared in over the years is mind-boggling. Softly Softly, The Persuaders, The Sweeney, Braveheart, Trainspotting, Troy, Wonder Woman, Chernobyl, and the recent TV adaptation of His Dark Materials. Over the years, he has blossomed into one of those performers whose presence helps to reassure viewers that things will be OK.

Cosmo’s latest role is on season two of The Bay, ITV’s Broadchurch-alike detective show. Again, he appears as a figure of authority, in this case the intimidating patriarch of a grieving family. So the obvious question beckons. Are you the murderer, then?

“You know that character in The Simpsons? Johnny Tightlips?” he replies, slipping into a perfect wiseguy impersonation. “I’m not saying I did, I’m not saying I didn’t. What? The Bay? I don’t know The Bay. Who?”

It turns out that Cosmo is fanatical about The Simpsons. “If I went on Mastermind – which is very unlikely – my special subject could only be The Simpsons,” he says with all seriousness. If you were wondering, his favourite episode is season eight’s The Springfield Files, and he also thinks they should stop making new episodes. “You know, I always look at the screen and when it’s the 4:3 format, when it’s not widescreen, that’s how you know it’ll be a good one.”

He isn’t done. “I was talking to Andrew Neil a while ago. We come from places that are very close, and were born within a couple of years of each other. He’s had an amazing career, but he once told me, ‘Jimmy, I am the man who brought The Simpsons to Great Britain.’ Back when he was head of Sky. He was so proud of that, more than anything else.”

Gradually, I try to steer the conversation back to my original question. On the opening titles of The Bay, his name appears after everyone else’s – “And James Cosmo” – so surely that means he’s the murderer. “Oh, that’s one of your conversations your agent has, he starts to laugh; a big, rolling contented chuckle. “Personally I’ve got absolutely no interest in that side of things. I wouldn’t care if they didn’t put my name on any credits at all. Actually, I would prefer if it said, ‘Such and such was played by Some Guy.’”

Cosmo isn’t a man who seems particularly weighed down by ego. This might be the sheer longevity of his career – you don’t get precious spending decades doing one-episode guest spots on Minder and C.A.T.S. Eyes – but it might also be down to the nature of his upbringing. Cosmo’s father was James Copeland, a jobbing actor who hung out with the glamorous likes of Sean Connery and Peter O’Toole in London, but he also kept “flitting back” to live in Scotland with his mother and sister. School in Clydebank didn’t sit well with Cosmo, something he now attributes to being “exceptionally thick” as a child.

“I had no interest in schooling,” he says. “I bunked off every Friday afternoon for a year. They didn’t know I was even meant to be in class. On Friday lunchtime I’d hide in the boy’s toilet and then, when they all went back, I would creep out and go fly fishing.”

In the end, Cosmo was caught and belted, before leaving school by mutual consent. “In retrospect, you realise oh, you know, I had a huge opportunity to learn stuff and become something greater than I am just now. But it just wasn’t me at that time. We’re like flowers, aren’t we? We all flower at different periods throughout the years.”

You suspect that he has flowered later. As well as trouble at school, he once missed out on a role in A Passage to India after having a “heated debate” with David Lean during an audition. But now he admits that “my fuse has got a lot longer as I got older”, and this is borne out in our conversation, rammed with references to his myriad enthusiasms.

Cosmo quotes Babylonian proverbs as easily as he does The Simpsons, and spends a sizeable chunk of the interview discussing George Monbiot’s book Feral as it pertains to the rewilding of Scotland. “Alan Bennett, he wrote this book The Uncommon Reader, it was very funny,” he tells me at one point. “And in it, he used this word, ‘Opsimath’. It means ‘one who comes to wisdom later in life’.” Would you consider yourself to be an opsimath? “I would like to, but I’ve got a horrible feeling I’m wrong,” he replies.

It’s interesting to look at the various stages that Cosmo’s career has been through. In the early days, his physical heft meant that he played dozens of police officers, then in the 70s and 80s he seemed to be tagged as a token Scot, playing characters with names like Jock McLeish and Glasgow McDade. Larger fame didn’t come until the 90s, thanks to roles working for directors including Mel Gibson and Danny Boyle.

And now we can add Bollywood to the list. Cosmo recently took a role in Jagame Thandhiram, a Tamil-language thriller that was shot in part in Faversham, Kent. “They approached me and my agent said, ‘Oh, you don’t want to do that.’ I thought, hang on a minute, because why not? Why not? Why am I so precious? I thought, ‘Well, I’ve never made an Indian movie.’ So I did it, and it was an astonishing experience, just fantastic.”

But it’s still Game of Thrones that gets him the most recognition. His run on the show finished in season three, but even seven years later it still follows him around. During lockdown he even made a video for Oldham council about the importance of wearing face masks, in character as Mormont.

“I didn’t read the books when I took the part,” he admits. “But I’m a fanatical fly fisherman, and I go over to my great friend in Washington state in America, and we fish for steelhead out there. Now Beau, my friend, is a real redneck. He doesn’t read anything and his writing is awful. Anyway, when I said I’d do this series, he bought the books. I don’t know how he did it, but he sat down and he read all of them. I made time to go over and see him, and we’d be fishing away, and he’d say: ‘Well, James, this has happened and that’s happened and you’re still alive, kid.’ I’d say: ‘Oh, good, good.’ Then one day he said: ‘James, they’re drinking beer out of your skull.’ So my death wasn’t exactly a surprise.”

The only downside of the role, he says, was that his character had taken a vow of celibacy. “In that show where everybody’s getting their kit off everywhere, I draw the short straw,” he sighs. Would he have preferred to have had a few sex scenes? “You know what? In my whole career, which is now like 56 years or something, I have only had a romantic kiss with one woman, in His Dark Materials. I’ve never had a love scene.” Should I use this interview to try and get you one? “Oh, I think it’s a bit of a wasted effort,” he chuckles.

The most delightful thing about Cosmo is how unshowy he is about his achievements. “I was in my mid-40s when Braveheart happened,” he says. “As you get older, you lose that testosterone-driven success thing. Am I going to be impressed by money or fame or whatever, really? I don’t think so, no. I mean, a day’s fly fishing is all I’m really aiming for.”

Cosmo brings up fly fishing so often that at one point I end up mentioning my dad, who is a flycasting instructor. “I’m appalled that you’re not a fly fisherman in that case,” he chides. “Listen, can we end this and just get your dad on the line? I think we could have had a much deeper and meaningful conversation had he been here.”

Sensing the slightest trace of sincerity in his sentiment, I start to bring the interview to a close. We’ve spoken for well over an hour, and Cosmo has been warm and expansive company. “I’m sorry I didn’t tell you much about The Bay,” he says as an afterthought as we part. “But at least now you can say that you spoke to Johnny Tightlips.”

• The Bay season two is coming soon to ITV