It took more than 25 years for Samantha Mathis to acknowledge how her life was shaped by one horrific night in 1993.

On 30 October that year, Mathis found her boyfriend convulsing on the pavement outside the Viper Room nightclub on Sunset Boulevard. Despite being told by the man he was with to “leave him alone … you’re spoiling his high”, she ran back inside to find his brother, who called 911. By the time the paramedics arrived, her boyfriend had stopped breathing. He was pronounced dead at the hospital, having overdosed on cocaine and heroin.

It would have been a traumatic event for any young person, but Mathis, who already had several movie roles, had to cope with an extra level of attention. The man she was dating was River Phoenix, who had already established himself as an alternative screen idol, thanks to roles in films such as Stand By Me, My Own Private Idaho and The Thing Called Love, a comedy-drama in which he had starred with Mathis in the same year that he died.

For decades, Mathis followed the lead of Phoenix’s family in keeping the events surrounding his death private, but recently she realised the story – and the pain that came with keeping it – was hers, too. So, on the 25th anniversary of his death last year, she spoke to the Guardian about her relationship with him and how she witnessed his death when they were both just 23.

“What came up for me last year around the anniversary was: ‘Oh, this also happened to me, and had a profound effect on my life, more so than I still understand even with all the years of therapy,’” she says. “I needed to talk about it for myself.”

As we sit together, barefoot, on her couch in Greenwich Village, her pain is still palpable, but she says there is, finally, an awareness of what she went through. “People who are in my life, but hadn’t been in my life at that time – and some people who were in my life then – were given a chance to understand how painful a time that must have been for me,” she says.

Still, she’s not sorry she waited so long to speak to the press. “By and large I was left alone, whereas now I would have been walking out of my apartment building with a camera in my face,” she says. “I can’t imagine a 23-year-old going through that today.”

With River Phoenix in The Thing Called Love. Photograph: Everett Collection Inc/Alamy

Mathis is now 49, and is rehearsing six nights a week for her role in the play Make Believe, which opens later this month in New York. “There’s great power in art to reflect back our humanity, and there’s something really exciting about doing that live,” she says. “There’s a nimbleness one must have. Just when you settle in and think that you know what you are doing, someone will drop a cue, an audience member will talk out loud, a light won’t happen or a prop won’t be there, and you are awake and alive to the moment.”

Since she was born in New York in 1970 to Donald Mathis and Bibi Besch, work – or acting, at least – has been central to Mathis’s life. Her parents divorced when she was a toddler and Besch, an Austrian-American actor, moved to Los Angeles with her young daughter to pursue her TV career. As a child, Mathis found that world “incredibly exciting and glamorous”, with frequent set visits and trips to the Paramount lot, where she would see stars such as Tom Hanks.

It was also a life of instability and heartbreak, and her mother was adamant, at first, that she should pursue something else.

She relented when Mathis was 16 and landed a spot on a TV pilot being shot in Australia – even as she continued to worry that her daughter would conclude that acting was an easy, jet-setting lifestyle. But Mathis wasn’t in it for the glitz. “I was in love with acting, so there was nothing she could say that would dissuade me.”

Later, she came to understand her mother’s warnings. “It’s a Gypsy life,” Mathis says. And, although that was part of acting’s appeal when she was younger, it was never easy.

To stave off loneliness, Mathis found comfort in relationships with other cast members – and many of her early romantic relationships developed on set. “The feeling of family, the feeling of community that occurs in creating a film or a TV show or a play, is something very familiar to me,” she says.

She dated Christian Slater, with whom she worked on her first movie, Pump Up the Volume, in 1990. She also dated Christian Bale when he was playing Laurie to her Amy in Little Women.

When she was younger, she says, she was more vulnerable to the possibility of falling in love with a co-star. “As you get older, you start to realise it’s a time and a place, and this person is not the character,” she says. “On-set romances don’t usually last.”

With Phoenix it was different. Mathis had sensed a connection with him years before they worked together, when he bummed a cigarette from her in a nightclub when they were 19. Being cast together later felt like an affirmation of the intuition she had had the first time she met him, with their romance developing from there. And, of course, they never had a chance to grow older together.

The 90s were a fraught period for Mathis, but also the source of many of her biggest breaks, with roles in Little Women (1994), The American President (1995), Broken Arrow (1996) and, a little later, American Psycho (2000). She says she was incredibly lucky to make the jump from television work to film acting as early and easily as she did. “It was the right role at the right time,” she says of landing that first big movie job opposite Slater. “The casting director really believed in me, and knew my work.”

She had been backpacking in Europe, and, when she walked in for the audition, the casting director gave her one look and told her to have a sleep before coming back. She did so, and got the job – and is aware, to her credit, that doesn’t happen for most people in Hollywood. “There were enough people who knew me because they knew my mother,” she says. “I think doors were opened for me much more easily than if someone from Iowa had just moved to Los Angeles.”

Samantha Mathis: ‘It has never been a better time to be a woman in her 40s in show business.’ Photograph: Christopher Lane/The Guardian

Mathis kept working after Phoenix’s death, but, a few years later, when her mother died of breast cancer at 54, she couldn’t keep it up. “It was just too much loss. I had to stop,” she says. “I stepped away from the business for two years. I fell apart.”

At 26, she had already been working for a decade. But taking a break from the film industry in your 20s is a fraught move for any actor, and particularly a woman. When she re-emerged, her acting coach suggested they try to find out what made her fall in love with acting, and “we started on a Shakespeare sonnet”, says Mathis. “Then we worked on other Shakespeare, and then I realised what I really wanted to do was a play.”

She also decided to use her profile politically, working with Amnesty International to help highlight femicide in Guatemala. Her father, a former aide to Robert F Kennedy and Lyndon B Johnson, had lived in Guatemala for a while – and in many ways her political work on femicide was the closest Mathis feels she came to winning his approval before his death in 2016.

“I said to him that I had never felt more like his daughter, and he said: ‘You’ve also never been more your mother’s daughter.’ And that was a huge moment for him to say that because in some ways I think he blamed my mother’s career for the demise of their marriage and had never fully resolved his feelings. It was an incredibly generous statement. He was acknowledging my work – and her.”

Mathis performed in her first play at 30, moving back to New York to pursue stage work. She had not lost her beginner’s luck, landing on Broadway opposite Jane Fonda in 33 Variations a few weeks after arriving in the city, and she has been appearing on stage ever since, although not exclusively.

Recently she has also played opposite Jim Gaffigan in the film Being Frank (2018), and appeared as a hard-charging businesswoman on the series Billions. On that show, she says, “I knew I was walking into a universe that was celebrating strong women and was excited about women who take no prisoners. It has never been a better time to be a woman in her 40s in show business.”

Make Believe, the play she is rehearsing, begins in a childhood playroom, before jumping ahead, and showing the grownup characters returning to the scene of their childhood after a death in the family. Mathis can’t wait to start performing later this month.

“That’s when the magic really happens?” I ask.

“Hopefully,” she laughs.

She gave up having strong expectations about work a long time ago. “It’s human nature to want more, but there’s so little that I can control. I have been making a living at it for 33 years, and that’s extraordinary.”

Make Believe is at the Tony Kiser theater, New York, from 30 July to 15 September