By David Hochman. Photographs by Steven Sebring.

Life's been a regular Mountain Dew commercial for Mathis lately. In the new action-adventure picture Broken Arrow, the 25-year-old actress -- previously known for dreamy, introspective roles in movies like How to Make an American Quilt and The Thing Called Love -- gets to go ballistic. She plays a park ranger who helps co-star -- and former real-life boyfriend -- Christian Slater save the world from an Air Force pilot-turned-nuclear war monger (played by that menace to society John Travolta). It's an intensely physical role, the kind that tends to make you a star in Hollywood. "They keep telling me it's a �Sandra Bullock role'," Mathis says with a laugh, something she and her best friend, Sandra Bullock, find endlessly funny.

Doing her own stunts in the wild back country of Montana might have been a bit traumatic, but it was cake compared with Mathis' personal life over the past few years. She was with her boyfriend River Phoenix that Halloween night back in 1993 when he died of a drug overdose on the sidewalk outside Hollywood's Viper Room. What normally would have been a difficult period of grieving was made worse by the often vicious press coverage. "I was disgusted," she says. "When River lay waiting in a funeral home for friends to say goodbye, someone snuck in from The National Enquirer to photograph him in his casket. And they ran it. How do these people live with themselves?"

Around the same time, Mathis' mother, actress Bibi Besch, was diagnosed with cancer and underwent several devastating rounds of chemotherapy. "The day the doctors told her the cancer was in remission," Mathis says, "was the happiest day of my life." Despite the dramas in her private life, Mathis managed to complete six film roles in the past two years, including recent turns as Michael Douglas' presidential assistant in The American President and as an American nanny who falls in love with a widower in the new British film Jack and Sarah. "The amazing thing about Sammy," says Bullock, "is that these last few years didn't suck the life out of her. What she's gone through with dignity is more than most of us could ever bear in a lifetime."

When you first encounter Mathis, she gives the impression of being quite fragile. She's small, lean and pale, like a Kirov ballet dancer. The only meaty part of her anatomy is her lips, which are so full that she can actually touch her nose with them when she puckers ("The boys in high school nicknamed me �the black hole' because my mouth is so big,' she says). But when she speaks, the strength of her voice lets you know that she's OK, even at moments when her voice cracks with emotion. "She's an incredibly strong girl," says Slater, now a close pal. "She's got a really good, solid core."

The tragedy of River Phoenix's death is obviously very near the surface for her. "I think about it every single day," she says, her eyes filling up quickly. She first saw Phoenix when she was 19: As she and then-boyfriend Slater arrived at a party, Phoenix was heading out the door. "I saw him, and I just knew," she recalls. When they worked together on 1993's The Thing Called Love (which also starred Bullock), it was Phoenix who did the pursuing. He told friends that his "head was going to pop off if hee didn't get to hold her hand." Mathis was equally smitten. "He was an ecstatic and happy person," she says.

Mathis admits that she "knew enough" about Phoenix's troubled lifestyle to know there was a problem. "I tried my best to help him," she says. "And for a while, I always had the thought, What if I had done this instead of that? Now I'm trying to get past that." She prefers to remember River at his best. "The nicest memory I have of him is when we went down to his family's property in Costa Rica, and we'd eat these mangoes for breakfast. River had them dripping all over him. When he ate, he ate with his hands. For me, he'll always be that beautiful, drippy mess."

Mathis' response to Phoenix's death was to shut down emotionally, to act like nothing had happened. She immersed herself in a hectic schedule, auditioning for roles and going out on a steady stream of dates. "It was my clich�d reaction to not wanting to deal with it," she says.

It was during the filming of The American President that she finally fell apart. "I just needed to unravel in order to be OK again," she says. "I resisted for so long, I was starting to feel schizophrenic. I was having panic attacks and just couldn't keep still." For nine months during and after the filming of the movie, Mathis lived with her mother, who was slowly recovering from her illness. "I'd just go to work and then come home and cook dinner for my mom, and we'd watch movies. It was finally time to stop running for a while." She also stopped dating. "For obvious reasons, I needed to be on my own for a while," she says.

Mathis emerged early last year with a new attitude. "I really got to know what's important in my life," she says. "I'm so grateful for my friends and my family. I think I was trying so hard to be sophisticated and cool, but now I've let it all go. Hey, I'm a nerd, and that's great."

IN MANY WAYS, MATHIS' EARLY LIFE PREPARED HER to deal with radical changes. Born in 1970 in New York City, she was 3 when her parents divorced. "I grew up in theaters and on location, following my mom around," she says. "I didn't have any sort of normality in my life." Despite the fact that both her mother (who recently played an older woman who has an affair with Larry on The Larry Sanders Show) and her grandmother (whose name was Gusti Huber and who was considered the first lady of Austrian theater some 50 years ago) were actresses, Mathis grew up hoping to become a fireman or a basketball player. "It's only recently that I've thought, Hey, I can make a living as an actress," she says.

Mathis played roles in TV movies nobody saw (American Nuclear and Cold Sassy Tree) before landing a juicy part opposite Slater as a perceptive high-schooler in 1990's teen drama Pump Up the Volume. Slater, who says her and Mathis became "good, good friends" shortly after the movie, recalls those days fondly. "We were just kids when we made that film," he says. "It's amazing to think she's living on her own now, paying her own rent and having a great career."

Not to mention a great time. Mathis is getting on with her life and seems genuinely happy with her life these days, which is no small accomplishment. Filming Broken Arrow in the wilds of Montana, she says, "was a great way to release some of the built-up aggression and energy. After doing so many roles that were emotional and after being so emotional, it was wonderful to do something where you just show up and have to react to the insanity that's occurring around you." Working with Slater again was also a comfort. "It's great to come to a set and already be at peace with the person you're working with," she says. "We can have a nice friendship now because we've both grown up." That doesn't mean they didn't act like kids at times. During a break from shooting one afternoon, the two got caught in a storm while hiking to a mountain waterfall. "There was lightning and thunder," Mathis recalls, "and I'm sitting there thinking: We're gonna die. This is it. Like, �CHRISTIAN SLATER AND SAMANTHA MATHIS DISAPPEAR SUDDENLY.' The only thing I had in my purse was Blow Pops. Of course, we made it out, but I have these pictures of us -- wet, exhausted, with bright blue mouths."

Working with Travolta was a bit more intimidating. "I knew I had to have my �nerd moment' with him," she says. "I had seen Grease 23 times the summer it came out, and I just had to tell him." One night during shooting, the cast got together for a party in the ballroom of a hotel in a small Montana town. There was a buffet, dancing and a DJ. "It was three days after I'd made my confession to John, and suddenly, Grease comes on. I turn around and John Travolta has dropped to his knees, and he's singing, �I got chills/They're multi-plyin'. And I'm right there with him, singing, �You're the one that I want/Ooh, ooh, ooh!'" She grins. "I think I can die a happy woman now."