Martha Plimpton may be the hardest working woman in show business. In the last year she's had recurring roles in television series, had a run on Broadway, and sold out a cabaret show at Lincoln Center. Plimpton's been on stage since she was 8 years old, and she shares her thoughts on how that helped – or hindered – her growing up. Plus: Plimpton performs Bruce Springsteen's "Thunder Road."                   Martha Plimpton performs "Thunder Road."