After auditioning for the part of hobbit Samwise Gangee, the loyal and stout pal of hero Frodo Baggins in the Lord of the Rings trilogy, in 1999, Sean Astin heard that “a fat guy in London” was his main competitor. “I was like, ‘I can be fat!’ ” recalls the 5’8″, 175-lb. Astin, who ran in the Los Angeles marathon that year. “I sent a note saying, ‘I’m committed to doing whatever I need to do to deliver this character-including eat like a pig.’ ”

True to his word, Astin, 30, packed on 30 lbs. and relocated to New Zealand in October 1999 for the 15-month shoot with wife Christine, 34, and daughter Alexandra, 5, in tow (the couple are expecting a second child this year). The can-do attitude didn’t end there. Evacuated from the set by helicopter one afternoon with a badly punctured foot caused by stepping on a stick, “he came back to work the next day,” says Rings producer Barrie M. Osborne. “He was a trouper.”

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Now his travails have paid off: With the success of Rings, which made $94 million in its first week, and two more installments to come (in 2002 and 2003), it will take more than an errant twig to block Astin’s career path. Not that he is unfamiliar with the hazards of fame. The older son of actress Patty Duke, 55, and John Astin (Gomez on the 1960s TV series The Addams Family), 71, Astin watched his mother battle manic depression before divorcing John in 1985. As a result his focus, says Astin, is to be a good parent: “My priorities are clear. I provide for and protect my family.”

That devotion was evident in New Zealand, where the cast filmed all three Rings movies simultaneously. “We became family for each other,” says costar Elijah Wood, who celebrated Thanksgiving 2000 with Astin’s clan. When it came to nightlife, though, Astin often opted out. “I was the old married guy of the hobbits,” he says. “They would go out clubbing. I was maybe good for one night a month of that.”

Even as a child Astin was the self-described “goody-goody” of the family, which includes brother Mackenzie, 28, also an actor. “Sean was extremely protective of me,” says Duke, who was diagnosed with manic depression in 1982 and whose rages included breaking her Oscar (for Best Supporting Actress for 1962’s The Miracle Worker) by throwing it across the living room.

Astin responded to the tumult by immersing himself in acting rather than acting out. In 1985 he won his first film role, in Steven Spielberg’s The Goonies. “He was still a kid, but he took it seriously,” says John Astin. But the complications were far from over: At age 25, Sean discovered that John was not his biological father. In her 1987 autobiography, Call Me Anna, Duke explains that in 1970 she’d had a fling with Desi Arnaz-Jr., then 17, followed by a brief affair with John. Soon after, the 23-year-old impulsively wed rock promoter Michael Tell, now 56. The Tell marriage lasted 13 days; by then, Duke was pregnant. (She married John in 1972.) DNA tests—conducted after Astin had a chance 1994 encounter with Tell’s niece, who suggested that he get tested—proved Tell to be his father. Now, though he considers John his dad, he is friendly with all three men—including Arnaz Jr., whom he met through John in 1995. To this day Duke maintains that Tell—with whom she says she was never intimate—is not Astin’s biological father. “I don’t buy that test,” she says. Astin takes a remarkably mellow view of the paternity muddle. “My feeling is that I want to honor my relationships with the people who looked after me,” he says—especially John, “who gave a huge amount of his time, energy and love.”

Forming his own family was far easier. At 19, Astin met Christine Harrell, then an assistant at his talent agency, and proposed within the first five minutes. “He was terribly brazen,” says Christine, who has been a producing partner with Astin. The couple, both UCLA grads, wed in 1992, just before Astin won the role of an overachieving, undersize college football player in 1993’s Rudy. To this day, Christine reports, “we’ll walk through an airport with 40 strangers chanting, ‘Rudy! Rudy!’ ”

He would prefer them to yell “Mr. President!” Conservative Democrat Astin, whose family shares a Spanish-style house in L.A. with a dog and two cats, says he may one day run for office—and has his sights on the top job. “I’m living a full life and I’m happy,” he says. “But I really am yearning for some new challenge.”