''The original thing about this film is that the acting is only one strand of a three-stranded piece,'' says British actress Justine Waddell (above, with Michael Weatherly), 27, of Peter Bogdanovich's The Mystery of Natalie Wood, in which she plays the late screen icon. Airing in March on ABC, the biopic combines original photos, interviews with such peers as West Side Story's George Chakiris, and reenactments of Wood's life and, as the title suggests, eerie death by drowning in 1981. Though Waddell nailed the American accent with the help of a dialect coach, singing was another matter. ''I sang, and then they dubbed rather like Natalie,'' she laughs, referring to Wood's West Side vocal stand-in. ''I have my own Marni Nixon.''