Julia Roberts and Lucas Hedges are just pretending to play a mother and her drug-addicted son in the drama Ben Is Back, aren’t they? The pair arrive at a bungalow in Santa Monica in clashing outfits, him in a blue-grey sweater and battered sneakers that used to belong to Shia LaBeouf, her in all black with a punky safety-pin necklace and rhinestone-studded boots. “Those are also Shia’s shoes,” Hedges jokes.

But they share an absurdist sense of humour, and a penchant for communicating with each other in growls. Their acting careers, too, have spawned from strikingly similar DNA: wide mouths, copper hair and an Oscar nomination by 22, her for Steel Magnolias and him for Manchester By the Sea.

“He was 19,” says Roberts, “a teenager.” Hedges raises his eyebrows. “Was I 19?”

Roberts nods. “I have a whole scrapbook on it.”

She is kidding, barely. Roberts was knocked flat the first time she saw Hedges in Manchester by the Sea, where he played an orphan juggling two girlfriends, the final year of high school and a less-than-ideal new guardian in the shape of an uncle played by Casey Affleck. “The nuance I felt that Lucas portrayed makes a very big impression,” says Roberts. “Only on me, apparently – I don’t think anybody else felt that way.”

Now, she is definitely kidding. Manchester By the Sea kicked off a two-year streak of great roles for the young actor: Lady Bird, Boy Erased, Mid90s, Three Billboards Outside Ebbing, Missouri, and currently, Ben Is Back, in which Hedges plays a junkie who ditches rehab to spend a tense Christmas with his doting mum, Holly, plus his bitter sister, doubtful stepfather and oblivious half-siblings. Ben is so determined to convince his family that he is healed, audiences can’t be sure when he is lying about his addiction. And Holly, who has managed to marry into money, has become so fixated on appearances, she is aching to believe everything Ben says. “If I can keep everything happy and optimistic, then going forward it’s all gonna be OK,” says Roberts. “And then I’m going to say: ‘See, I was right. He’s great – he’s better.’”

Facebook Twitter Pinterest Roberts and Hedges in Ben Is Back. Photograph: Lifestyle pictures/Alamy Stock Photo

Together, their characters make for an unusual duo, a mother and son charming each other as though on a blind date. Which is a lot like how both are in person. At 22, Hedges is eight years older than Roberts’ eldest twins, and seems to slot neatly into place as both a dry run for her teenagers’ pending young adulthood, and as a talented peer. They trade slow-motion high fives and stroke each other’s faces when comparing bone structure, Roberts’ hologram manicure glinting on Hedges’ pale skin, while Hedges swoons: “I wish I had her cheeks – Jesus Christ!” When he casually turns to Roberts and smiles, “You’re my universe,” she rewards him with her high-voltage grin.

At 51, Roberts is secure in her standing as her generation’s biggest female movie star. Her last public gig was presenting the best picture award at the Oscars, a moment bedevilled in recent years by politics and envelope mishaps, but to which Roberts, wearing a bold, plain pink dress, brought a calm authority – even when the prize, which was wide open, ended up going to Green Book. The envelope “could have said anything,” says Roberts. “I just thought: ‘I hope the font is big enough.’ But it’s fun to just come in at the end and, you know, change somebody’s life and then get home before the kids are asleep.”

When Roberts read the screenplay for Ben Is Back, she told the writer-director that she wanted Hedges to play her son. But there was a problem. The film-maker, Peter Hedges (who co-wrote the script for About a Boy, and the novel What’s Eating Gilbert Grape, adapted into the Leonardo DiCaprio film), is Lucas’s father.

“He’d been pretty clear he didn’t want to do a film with me,” says Peter. “And while I didn’t necessarily like that that was the case, I respected it and I was never going to ask him to be in it.”

Facebook Twitter Pinterest Roberts in Steel Magnolias, 1989. Photograph: Everett Collection / Rex Feature

Lucas wanted to prove he could make his own career. Plus, he admits, having his father talk to him intimately about one of his characters is: “Ughhhh, so gross! A somewhat graphic, overly detail-oriented conversation about something that is my private life? The only other thing like that is sex.”

Still, Roberts pursued her casting idea, even texting Peter a selfie with her son Phinnaeus, that read: “See how good I am with red-headed boys?” Finally, Lucas came round to the idea. For Roberts’ first meeting with her new fictional child, she invited him to move into a guest bedroom in her home in California and come along to her kids’ soccer games. “I thought I needed to bridge the gap between the family that I have and am devoted to, and the family that I’m about to join,” she explains. “You really want to know who you’re getting into the dark of night with. And I think that we were all pretty clear on who we all were by the time we got to New York.”

Lucas was mine during all that time. I never felt I was competing with Peter as a parent

Once on set, the family lines were redrawn. Peter wasn’t a dad – he was the director. The Hedges rarely spoke outside of work, and treated each other with such formal congeniality that the Christmas holidays felt strange. “It was so bizarre,” laughs Hedges. “Christmas morning with my director? I have to get him presents? He’s giving me presents? So weird!”

“The thing that was really masterful about what Peter did for me, which I am really only able to fully appreciate from a distance, is Lucas was mine during all that time,” says Roberts. “I never felt that I was competing as a parent, which I think is pretty exceptional.”

As Ben is Back’s disastrous homecoming visit wears on, Roberts’ Holly gets increasingly frantic about keeping her son safe – which means protecting him from himself. In one scene, she bursts into a dressing room because she fears he has hidden drugs in his shoes. Later, she drags him from their SUV to pick out the burial plot he will need if he doesn’t stay sober.

“I remember pulling you out of that car so intensely, just thinking that this could really change our relationship,” says Roberts. “I just remember in Mystic Pizza, I had to slap Annabeth Gish, which I did not want to do, and it was so hard to actually slap a person.”

“Whoa!” says Hedges. “You literally slapped her?”

“And I felt it took weeks for us to recover from that,” agrees Roberts. “Both of us. I was so wracked with guilt, and I think she felt so ... slapped.”

Facebook Twitter Pinterest Hedges with Casey Affleck in Manchester By the Sea. Photograph: Allstar/AMAZON STUDIOS/STUDIOCANAL

“Can I ask you a question?” says Hedges. Roberts turns to give him her whole attention. They link thumbs.

“There is a part of me that really wants to value my life in terms of my career – and I am doing everything in my power to go in the other direction, like in terms of establishing self worth in everywhere other than on set,” says Hedges, stammering a bit. “I’m just wondering, what would you offer me with respect to, like, living in my fullest potential?”

“When you come over to the house, I’ll tell you,” replies Roberts.

Thirty years ago, she continues, “I was Lucas’s age. A lot has changed in the industry.” Mobile phones, the internet, streaming video. When she was the rising star, the path to Hollywood success had more clearly defined steps. The right role – say, Pretty Woman – could turn you into an international superstar overnight.

“I don’t really think you could make that movie now, right?” asks Roberts. “So many things you could poke a hole in, but I don’t think it takes away from people being able to enjoy it.” Regardless, even getting the role of bubbly sex worker Vivian Ward was luck. The part only came to Roberts after at least eight other major actors turned it down. “It really is not a measure of talent, particularly in the beginning. It’s a measure of good fortune – and being able to have your wits about you enough to make something out of that good fortune.”

And having the sense to recognise when you almost made a mistake, such as when Roberts nearly turned down Notting Hill. “I did not want to do that movie,” says Roberts. “The pitch of it sounds awful, right? Do you want to come and play the world’s biggest movie star and then fall in love with the bookshop owner? No!” But she met the film-makers anyway and liked them so much, she shrugged. “‘What the hell?’ I, you know … And it’s this great little jewel of a movie.”

“It’s so much luck,” agrees Hedges. “It’s like getting the golden ticket. Is there something about Charlie and the Chocolate Factory I don’t remember? Does he just win the golden ticket through luck, or is there, like, a chosen-one aspect?”

“I think he just proved that the more candy you eat, the better your odds,” says Roberts.

As of now, Hedges has gorged himself on serious roles. He has specialised in troubled boys, be they addicts, meatheads, mourners or closeted young men. “I’ve told a lot of stories with the weight of life itself on my shoulders, and I don’t need to do that any more,” admits Hedges, “At least for a while, until I feel the urge come back.” Maybe it’s time to follow Roberts’ path by embracing the romantic comedy?

My God, the last thing I’d want to do is do a sexually fluid romantic comedy with my father

“Hell, yes – in a heartbeat!” says Hedges. “Even in my apartment right now, I’m like, Marie Kondo-ing it, getting rid of everything with dark energy, and just clearing it all out for this romantic comedy to come down the aisle.” Part of his mood change is because he is in love. “I’m in my first ever long-term relationship and that’s the best and biggest thing in my life right now. Intimacy! Let’s figure that out.”

Hedges once described himself to Vulture as: “Not totally straight, but also not gay and not necessarily bisexual.” He has been open about his sexuality existing on a spectrum, terrain that romantic comedies are only beginning to explore. Roberts has a brainstorm.

“Let’s call Peter Hedges! He’s a good writer!” she exclaims.

“My God,” groans Hedges. “The last thing I’d want to do is a sexually fluid romantic comedy with my father.”

“Just invite me to the set,” says Roberts, with a sinister chuckle.

As for Hedges’ girlfriend, she jokes he looks like Prince Harry. Does he see the resemblance between Roberts and Meghan Markle, who claims she was inspired to act when someone told her she looked like the famous star?

Hedges looks at Roberts and gasps. “I see it a lot! There’s a structural similarity.”

“But you know what that would mean?” counters Roberts, remembering how earlier we discussed their similar looks. “Based on this conversation, you look like Meghan.” They both laugh. “I feel a TV movie coming on!”

Ben Is Back is on general release.