Mary Steenburgen is sitting on the porch swing of her ivy-covered house outside of Santa Barbara, gazing at her white picket fence, spotted dog and two blond children. In a scene that could have been lifted from Parenthood, the new smash movie in which she stars, Steenburgen, 36, gets up from the swing, picks her way past the toys and other kiddie litter and goes into the kitchen to throw a pan of popcorn on the stove. Minutes later the smell of scorched kernels fills the air. "Oh, I think I've burned it," Steenburgen groans. "What a mother I am. I can't even make popcorn."

Motherhood and Parenthood have been foremost in Steenburgen's mind lately. In the bittersweet comedy directed by Ron Howard, Steenburgen plays Karen, the suburban earth mother around whom the problems of family life swirl. As her husband, Gil (Steve Martin), and other members of the movie's extended family lurch from one crisis to another, Steenburgen's Karen remains at all times the grounded, sensible parent. When Gil complains about the behavior of the couple's three children, Karen replies, "They're kids, not appliances. Life is messy."

It's a line Steenburgen could have delivered without a script. Separated from actor Malcolm McDowell, her husband of nine years, and still mourning for her father, Maurice, who died during the filming of Parenthood, Steenburgen has spent the last year or so putting her emotional house in order. Fortunately, she has her children, Lilly, 8, and Charlie, 6, and the success of her new movie to buoy her. "I'm actually in a film that is a big box office success," says Steenburgen, knocking superstitiously on the wooden arm of the porch swing.

Even if it weren't reaping vast quantities of yuppie coin, Parenthood would represent a milestone in Steenburgen's career. Her part as Martin's ever cheerful wife, combined with her role as the aging beauty queen Elain in last spring's Miss Firecracker, marks her return to comedic roles after years of being miscast as overly sober women in such uncommercial period pieces as Ragtime, A Midsummer Night's Sex Comedy and Cross Creek. And while her portrayals of Karen and Elain may not win her another Oscar—in 1980 she was named best supporting actress for her performance as Lynda Dummar, the ditz with dignity in Melvin and Howard—Steenburgen is more concerned with getting her film career back on the laugh track.

"Mary has a real ability for comedy," says director Tommy Schlamme, who cast Steenburgen in Miss Firecracker. "In contemporary cinema, you're sexy, you're serious or you're funny. But to put all three of those together in one person is rare for women in films. It's what Mary has a great ability to do."

Steenburgen developed her knack when she was growing up in North Little Rock, Ark. Her father, a freight train conductor, suffered a heart attack when Steenburgen was 6. As a result, Mary, her mother, Nell, now 66, and sister Nancy, 31, were always terrified he'd suffer another one. "I know that's why I became an actress," says Steenburgen. "In my dream world I could get mad and scream and yell, and if somebody died, they got up again. In real life, I didn't dare try it."

Mary was a gangly teenager who mastered the three R's but lacked the requisite three B's—blond hair, blue eyes and big breasts—to become a true Southern belle. "Anytime I had a date, it was at the Sadie Hawkins Day dance," she laughs. After spending a year at Hendrix College in Conway, Ark., in 1972, she set out for New York to try life on the other side of the Mason-Dixon line. In the Big Apple, the girl who felt so out of place in Arkansas quickly blended in. "New York had this wild beat that anybody could dance to. It was very nurturing to young people." Mary studied acting at the Neighborhood Playhouse and waited tables to pay the bills.

In 1977 Steenburgen moved to Los Angeles, after Jack Nicholson picked her over more established actresses to play the spitfire spinster in Goin' South, a film in which he starred and directed. The following year she made Time After Time with Malcolm McDowell, the British actor whose credits included A Clockwork Orange and Caligula. The co-stars fell in love, and after McDowell divorced his first wife, they married in 1980. Over the next five years, Mary made five movies, two of them while she was expecting. "Both my pregnancies are documented on film: Lilly in Ragtime and Charlie in Little Red Riding Hood" says Steenburgen. Preferring to tend to her children rather than accept roles that weren't to her liking, Steenburgen took a hiatus from the movies after her son was born.

"I am not a workaholic," Steenburgen explains. "I don't know how you do that and be a mother. I feel stretched to the max to do two films a year." When Steenburgen needs advice about her parental juggling act, she goes to the source, her mother, who worked as a school secretary and for years was the sole supporter of the family when her husband was ill. "I've asked her, 'How did you do it? How did you make it look so easy?' My mother would come home from work and somehow make us feel so immensely cherished. It must have been at a great cost, but she never showed it.

"The only thing my mother did that I swore I would never do to my children was make me wear mother-daughter dresses," Steenburgen laughs. "And since my father never had a son, he used to make me play golf with him. Mother-daughter dresses and golf were about the worst things they did to me."

Word of her father's death reached Steenburgen at the Atlanta airport while she was en route to his side in Arkansas from the Parenthood set in Florida. "It's really hard to get that news in an airport with strangers all around," says Steenburgen. "We all lived in fear of his death. Luckily, I ended up having an amazing relationship with my father. It's interesting that his death occurred while I was making the film where I was most thinking about my parents."

Steenburgen says she isn't the paragon of motherhood that she portrays in Parenthood, but her devotion to Lilly and Charlie belies that. "I'm intensely respectful of my children's privacy," she says. "Kids have a hard time dealing with [separation] without having to read about it." However, she allows that her marital troubles don't make her a single parent. McDowell, who lives nearby, "loves [the children] very much and spends a lot of time with them," says Steenburgen. "He is an incredible father, and they have a special, wonderful relationship with their daddy."

When Steenburgen sits on her front porch talking about her children, the image easily blurs with that of her onscreen counterpart in Parenthood. And the more she talks, the harder it is to distinguish between the two. "My character's name could have been Faith, because that's what she is really all about: faith in her husband, faith in her children, faith in herself," explains Steenburgen. "For her, life holds so much promise. She's different from her husband. She lives in a more serene place than he does. I understand her and I know what she is all about."

—Mary H.J. Farrell, Vicki Sheff in Los Angeles