Come with me on a journey back to the early 90s. To bleached denim and bare midriffs. To when Beverly Hills 90210 was the biggest teen show on Earth and Jason Priestley was its biggest star. His character, Brandon Walsh, was the perfect all-American high-school heartthrob: white T-shirt, big quiff and eyes to launch a raft of terrible ocean-based metaphors. When I was 11, my friend had a lifesize poster of Priestley on her wall. Now here he is: lifesize and sitting with me in a hotel bar in Glasgow, where his directorial feature debut, Cas & Dylan, is screening at the city's film festival.

Priestley is wearing a flat cap, argyle sweater and grizzly beard. "I was very much Team Dylan," I blurt, referring to the bad-boy character played by Priestley's 90210 co-star, Luke Perry. "Oh! My sister was Team Brandon, though."

Older, stockier, but still handsome, Priestley handles my ungainly opener with good grace and conversation takes a smoother turn to Cas & Dylan. "I've been directing for 20-plus years," he explains, reeling off multiple TV credits bookended with his two biggest hits: Beverly Hills 90210 and HBO Canada's Call Me Fitz. He's proud of his first feature film but it's clear that its making was not without anxieties. "I won't lie to you, I struggled with the set up and I struggled with the first act. I struggled when it was a script and I struggled with it after I shot it and when I was editing for months. It took a long time to get the movie to a place where I didn't lose the audience in the first act."

An odd-couple road movie, the film stars Richard Dreyfuss as Cas, a cantankerous old coot with an inoperable tumour who sets off for his cabin for a last hurrah. Tatiana Maslany's Dylan is his foil; the ultimate Manic Pixie Dream Girl – the archetypal kooky young spirit who comes along for the ride. Dreyfuss spent two and a half hours on the phone to Priestley before he agreed to sign on. Was the director ever nervous about giving notes to the veteran actor? Say, for example, in the scene when Cas gets high on drugs?

"Are you kidding me?" Priestley's laugh is a ratatat ha ha ha from the back of the throat. "You think I need to explain to Richard Dreyfuss how it feels like to be high on anything? He's Richard Dreyfuss."

After swerving on to the subject of why the appeal of road trips endures (Priestley doesn't have a favourite by the way, pointing out, "I can't say On the Road because everyone loves Kerouac, it's too pedestrian"), I figure it's finally time to ask the important question: was Shannen Doherty really as bad a nightmare on Beverly Hills 90210 as everyone said? "Ha! Oh, Shan." He's giving nothing away.

If Priestley's Brandon was 90210's moral conscience, the mum-pleaser, then Dylan (Perry) was the wrong 'un. You knew that because he had a leather jacket, a Harley and full ownership of "the squinch". It's time to up our game.

Can I have Luke Perry's number?

"Ha ha ha ha ha." Lots more throaty laughter now. "He's too old for you! He's an old man, he has that old man neck thing now."

Saggy?

"It's kind of leathery. Too much time in the sun."

I mime Botox needles.

"He's a cowboy, he's about being rugged. He'd never do that."

Priestley's pre-90210 pin-up credentials were impeccable: Jim Stark in a high-school production of Rebel Without a Cause, 21 Jump St with Johnny Depp, the lead in TV series Teen Angel. When filming began on Beverly Hills 90210, he was 21 (playing 16) and perpetually mobbed by screaming girls. One Christmas, early on, Priestley holed up in Switzerland for "complete and total anonymity". By 1994, he was the only cast member to have been nominated for a Golden Globe, while Beverly Hills 90210 had become the most-watched show in the US by viewers under the age of 35.

"When I look back, I am surprised I made it out. It was hard, being a young man in LA … I never had the temerity to consider that [level of fame] was a possibility. Nobody would expect that; you'd be an idiot to expect that."

Priestley does his best not to sound bitter – it doesn't fit with his practised, good-guy, upbeat charm – but there is a long pause when I ask for his most memorable, or surreal, experience of working on 90210. "When I left the show, it was so anticlimactic, it just left a bad taste in my mouth." He grimaces. "It was the fourth episode of the ninth season. I did the first scene of the morning – literally with this actor who was brought in to replace me – and that was it. I hugged the crew, picked up my box of stuff, went to my car and drove away. There was no party, no nothing. I felt like I'd wasted nine years of my life."

In the world of teen TV, where Freaks and Geeks now enjoys a cult legacy and critical adoration for My So-Called Life and Party of Five remains, Beverly Hills 90210 was the bombastic outlier. It pulled in more viewers (20m per week at its peak) than those three teen shows combined. Critically, it bombed hard after the first couple of years but still simpered through 10 full seasons, launching Hilary Swank's career in the process. The 2008 reboot, which arrived with a new cast, lasted half as long.

"That show never reached the heights of success our show did, not even close, it pulled just 1m viewers." Priestley was invited to direct an episode and couldn't get over the disparity in production. "They had all the toys and all the money. They shot in a real studio; we shot in converted warehouses with rats everywhere. Our show was the most ghetto show ever. To watch it, it looked glitzy but it was totally ghetto. We shot so far away from Beverly Hills we were shooting in El Segundo."

Yet, Priestley admits, "It's a much more dangerous world for a young famous person now, it's mayhem. You can understand why so many of them have bodyguards." But the new 90210 kids wouldn't get away with a fraction of the off-set behaviour he and his co-stars did either, I suggest. "That's not my problem – that's Bieber's problem. I'm 44 years old, happily married with two beautiful kids. Thank God it's not my problem!" Plus, he points out: "It was Shannen who was out partying, getting in bar fights – not me."

That said, there was a moment at the peak of his success when Priestley cruised around LA, rotating his dozen cars – "Yeah, 12 cars and only one ass. I learned that lesson." The petrolhead passion later translated into a short-lived career as a race-car driver and commentator – but for an accident on the track in 2002 that nearly killed him. "We don't have to talk about that," says Priestley, the only time he seems uncomfortable. "A thing like that has to change you in some way. If it doesn't, I think there's something wrong with you."

Time, then, for my second most important question: did any of them ever imagine that weedy little David Silver (played by actor Brian Austin Green) would end up marrying the world's hottest woman (Megan Fox)?

"I'll tell you a secret about young Brian – he always did very well with the ladies. He's a great guy. What's the magic? I don't know, I'm not his type."

Who took the most advantage of the attention? "I don't think any of us took enough advantage, to tell you the truth. We were way too good; we should have been way worse. I had a girlfriend a lot of those years; Luke was married."

Is there an annual meet-up over brunch? "Ha ha ha. No. Luke and I are the most friendly and try to get together a couple of times a year."

So, about that phone number …

The Glasgow film festival runs until 2 March.