When Jill Clayburgh was cast in NBC's Reason for Living: The Jill Ireland Story, the contrast intrigued her: Clayburgh has conducted her life privately, while Ireland revealed intimate details of her cancer and her son's drug addiction in two autobiographies. (The second, Life Lines, inspired the film.) ''I really tried to wrap my brain around it,'' says the two-time Oscar nominee. ''It's so unlike my instinct to reveal things, and Ireland was on The Joan Rivers Show two weeks after her son died. When she was dying herself, she was right out there. I could never do that to myself, but I know it fed her.'' NBC's production faced troubling obstacles: Both Ireland, who had intended to play herself, and her son died before filming began, and Ireland's husband, Charles Bronson, subsequently sought to block the project. Executive producer Bonny Dore says that the dispute was settled ''amicably,'' and Ireland's presence in the movie was strong: She's credited as co-executive producer, and Clayburgh drew on hours of videotaped interviews with her background. ''She was close to death,'' Clayburgh notes. ''But at a time when most people would have said, 'Hand me the drugs' she was passionately involved in the script.''