In a quiet corner of his Los Angeles home, Dermot Mulroney grapples with a question that many actors’ egos would not allow them to entertain: why isn’t he a bigger star?

“Well, I had some alcoholism. That slowed me down. And I ... wasn’t six feet. Does that work? No, that’s a little flimsy. Let’s keep thinking.”

To be fair, few actors have been as consistent a presence on the big and small screens. Although he is best known for the 1997 romcom My Best Friend’s Wedding, Mulroney is as capable of grit as he is fluff, with film roles that include the Aids-themed drama Longtime Companion, the serial-killer thriller Copycat and the suburban angst of The Safety of Objects. More recently, he has found steady work in television, appearing in New Girl, American Horror Story, Arrested Development, Enlightened and the US adaptations of Shameless and Four Weddings and a Funeral. This month, he joins the second season of Hanna, the high-octane Amazon series about a child assassin.

He’s hardly a failure, then. But you might have expected Mulroney to rise higher, given that he says he was “hand-selected” for stardom. In 1985, the year Mulroney turned 22, a new movement was sweeping through Hollywood with St Elmo’s Fire and The Breakfast Club: the Brat Pack of twentysomething A-listers, led by Emilio Estevez. Mulroney found himself caught in their slipstream as talent scouts sought out more young blood. The same applies to most US actors now aged between 54 and 58, he says (he is 56). “It’s Johnny, Brad and Keanu – the list goes right down the length of my arm because they were pulling in all of those 20-year-old kids that year, and all of them were good.”

As the middle child of five growing up in Alexandria, Virginia, Mulroney acted in “back yard” theatre and school plays, although his chief creative pursuit was the cello. (He played in the scoring orchestra for Mission: Impossible III, among other blockbusters.) Later, while studying film at Northwestern University in Illinois, he appeared in the comedy revue – but his plan, after graduation, was to work as a cameraman in Chicago.

When an agent came to audition young actors, Mulroney, on a whim, put his name down. “I was convinced that it was a waste of time, a pipe dream, and not worth any actual attention or effort – I wasn’t about to do that,” he says. “Two minutes later, I was signed.”

He was cast in his first role, the television film Sin of Innocence, after only four months of auditions. “Then I took off like lightning. Once I got signed, I thought: ‘I’m really going to do this, and I’m going to do it for ever – I’m going to make sure I don’t blow it.’” Meanwhile, Mulroney’s agent was billing him as a “better actor than Tom Cruise”: “He’ll be a household name.” (Actually, Mulroney says: “It sure made things hard, ‘Dermot Mulroney’ did.”)

The publicity machine kicked in to a higher gear in 1988 for Young Guns, a “Brat Pack western” starring Estevez as Billy the Kid, alongside Charlie Sheen and Kiefer Sutherland. The newcomer Mulroney was marketed as the fifth of “six reasons why the west was won”. “That’s that predetermination, where I was hand-selected.”

Cutting loose on the hotel bars and steakhouses of Santa Fe with the young cast was “a top-favourite experience”, Mulroney says. “We were all drinking voraciously in a way that I don’t think young actors do today. I think that took on some legendary status.” The members of the group soon learned “who could handle their liquor, handle their scandals”. What scandals? Mulroney becomes vague. “Charlie Sheen, climbing over a balcony, kind of half-clothed …”

As for Mulroney’s own drinking, he was never really bad on set and didn’t let people down, “except for” – he counts, then holds up four fingers to the camera – “this many times. But it’s a nailbiting business, that’s what I mean. Every performer is in a state of stress even when they make it look easy.”

Young Guns was a hit at the box office, and in the home-video boom. But his best-known role remains My Best Friend’s Wedding: an “insidiously hilarious” film that people still feel affection for 23 years on. “A lot of these things are welling back over with nostalgia, and I’m all in with it – but it’s a real feeling.”

Indeed, some viewers remain “deeply troubled” that his character rebuffed Julia Roberts’s: “That’s what makes the movie great,” he says. The two remain good friends and occasional collaborators, having worked together on August: Osage County and Amazon Prime’s Homecoming. “I’ve watched her family grow up,” he says. (He has two daughters, aged 11 and 12, with his wife, the singer Prima Apollinaare, as well as a 21-year-old son from his first marriage to the actor Catherine Keener.) He heaps praise on Roberts’s “sensible household”: “They use cloth towels in the kitchen, she makes her own focaccia pizza with vegetables from her garden … she’s got chickens.”

While My Best Friend’s Wedding relaunched Roberts’s career, it combined with 2005’s The Wedding Date to typecast Mulroney as a romcom man – he believes, unfairly. “I only did two; they just happened to be fucking great. And romantic comedies are indelible, so they made a deeper impression I couldn’t escape.”

Yet Mulroney’s biggest footprint on popular culture may just be a 2012 Saturday Night Live skit about his being confused with the actor Dylan McDermott. (“It just goes on and on,” he marvels, without bitterness.) Just three episodes of Friends in 2003 – he played Rachel’s cocksure maternity cover Gavin – had a similarly outsized impact. He declined three more: “There’s a regret.”

During the 2000s, Mulroney turned down most television in favour of the integrity and variety of independent film. But with the 2008 recession, indie budgets stopped stretching to relatively high-paid actors “to play the dad”, he says. “It took me a couple of years to realise: ‘I am not making a living any more the way I used to.’” TV, however, was booming: “I flexed a little, and maintained working status.” Today, Mulroney suggests, “an actor who doesn’t say yes to everything is really making a mistake”.

For Mulroney, 35-odd years have passed “one job at a time”. “There is a point, I never got there, where you can plan your near future, people want you, and the bucks are big.” Yet that was what he had been on track for, when he entered the industry. Does he feel that he should have made the A-list? “The straight answer is yes, of course,” he says, slowly. “I thought I was just going to build right up and I’d be a top movie star.” Mulroney seems to have surprised himself with his answer. “I’ve never even really said that. But that was the programme. There’s no denying, that was gonna be the plot.”

So what happened?

That is the question that starts Mulroney spitballing: not obvious enough a leading man? Stiff competition? (“People like Patrick Dempsey, man, they were tough to beat!”) Too handsome! (“I heard Rob Lowe say that – but have you seen the guy?”) “Really, the answer is, I don’t know,” he finishes. “I tried fucking hard.”

But he didn’t exactly fail. “I had a fair shot at the welterweight belt, I took a few on the chin and I’m still in the game – so I couldn’t be happier. I could have made more money. But I couldn’t give a damn about being more famous.”

He has seen the toll of that first-hand. Mention of Lindsay Lohan, his co-star in Georgia Rule in 2007, prompts a visceral reaction: “She’s one of the first victims of social media.”

“My hands are shaking,” Mulroney says, showing me, “because these are traumatised people. People her age, they were hit with the first wave of being famous in that way. We’ve learned since how destructive and awful that is, especially if you’re young and seeking it deliberately.

“Now she’s like, an elder stateswoman – and we’ve seen them go down, one after another.” He won’t name names: “So many people still have so much time to recover from the pressure. But it’s been devastating, devastating, to watch. “So be careful, you kids, if you think you want to be a movie star or on a TV show or be in some band – fucking think twice! You can take the F-word out, but I mean it. It’s unsafe.”

Unsafe to pursue fame? “Yeah, until now.”

It is quite an extraordinary speech, and not always clear where Mulroney is locating the danger for young performers: the mental health toll of show business or the predators operating within it.

It is “that sense of being watched at all times that drives celebrities crazy”, he believes – exacerbated by social media and reality programming. “The pressure of fame, the type of commitment it has started to take, of showing people so much of your home life … it turned all of our cultures.”

But long before the selfie generation, Mulroney says, “Corey Feldman and those guys, just younger than my generation, were horribly abused by this industry”. And before them, “it was all the 70s sitcom kids, who were so horribly used up”.

“How many made it out alive, not to mention sane? I don’t know what to do, but I can’t watch it any more.”

Pressed for specifics, Mulroney is clear: he has never witnessed a sexual assault or “anything overt on any set” (nor does he count his cohort among “the traumatised crowd”). “But I know, it’s a lurking thing.”

He hopes that it is changing: #MeToo has already “cleaned up our industrial worksites, for people’s safety and sanity”, he says. Young actors today “won’t ever have to have that creepy feeling, if not overt, misogynist, sexist, sexual assault and beyond. Those people are up and running, and on the ground; they’re starting to be able to bring people in behind them.”

He is also excited by the creative opportunities that may arise from “our current collective experience” – both the Black Lives Matter protests and Covid-19. In March, he had been in New York, finishing shooting on a season of his Fox crime drama Prodigal Son – resulting in a “nail-biting” few days of coast-to-coast travel before lockdown was imposed. “That was one of the last shows working, and I plan to be on one of the first shows going back.”

That is why he has been so serious about quarantine, Mulroney says, brandishing a green bandana that has been serving as a mask (which he has fiddled with throughout the video interview). “My best contribution is to be able to reopen the industry and that huge chunk of the California economy.”

He frets that he is underdressed – a polo shirt, worn under a hoodie, over a T-shirt – for the photoshoot later. “Does it look a little disrespectful, with the hoodie? What do you think? I might have to put on a real shirt, right, not, like, a golf shirt.”

He has appreciated the family time that has been enforced by coronavirus. “I think it will go down in history as ‘that time when you spent all that time with your dad’.” The “secondary area” of his home, from which Mulroney is speaking, has been serving as his daughters’ classroom. (It features a large oil painting and a cushion shaped like a dog’s face. “The little glimpses you see of people’s home interiors now,” Mulroney says, peering at mine.) But he is keen to get back to work. “We’re going to release this creative force on ourselves. I very much want to be a part of what’s next.”

As he said earlier, consistency brings its own rewards. “I never did reach that apex of my class, the No 1 guy. Therefore, I’m much more usable now. And there are fewer of us. Sometimes, it is a matter of outlasting.”

• The second series of Hanna is on Amazon Prime