Since his first film role in 1983's Testament, 15-year-old Lukas Haas has proven to be more than a flash phenomenon. From his Amish boy in Witness (1985) to his praised performance in the TV movie The Ryan White Story (1989), Haas has shown a depth and maturity unusual for a child star. His role as Buddy in Rambling Rose has allowed the actor another departure. ''I'm different than I usually am,'' he says. ''This time I'm not an innocent little boy.''
No, in fact, Haas' big scene is one in which the curious Buddy gets a tactile lesson in female anatomy in the bedroom of costar Laura Dern, who plays Rose, the family baby-sitter. Being the self-taught actor that he is, Haas delved into his own method for the segment. ''I wasn't trying to act,'' he says, sucking on a virgin strawberry daiquiri in a Los Angeles restaurant. ''I was just trying to get through the scene. It was pretty rough. If Laura or I made a mistake, we'd just stop for a second and go on.''
In real life Haas is that Hollywood hybrid that falls somewhere between adult and child. He lives in Austin, Tex., with his mother, Emily Tracey, an opera singer-turned-screenwriter, his father, Berthold Haas, a painter, and his twin brothers, Nikolai and Simon. ''Bobby'' Duvall counts as one of his best friends; Laura Dern is another. And when he's not on location, he plays drums and piano in a band called the Rogues. ''Basically,'' says Tracey, leaning in from a nearby table, ''our house is like Rose's we have ducks and chickens and doves and a dog and four cats.'' ''Except,'' adds Haas, washing down his cheeseburger with a chocolate shake, ''we don't have a baby-sitter like Rose.''