How did Julie Warner feel as she emerged from a pond stark naked to startle a contemplative Michael J. Fox in the first scene of her feature film debut? ''Cold!'' says Warner, who plays Lou, the wholesome Southern darling who derails Fox from the fast track in the new romantic comedy Doc Hollywood. ''Humiliation aside, and I'm not an exhibitionist, the hardest thing about it was that I was freezing,'' she explains. ''Lou represents earthiness, and it wouldn't have been right for her to be in a Speedo.'' Still, the 26-year-old New York-born actress, who now lives in L.A. with boyfriend Hank Azaria (a regular on The Simpsons), admits losing her cool when watching the nude scene for the first time: ''I was sitting next to (director) Michael Caton-Jones and he said he had claw marks on his arm.'' But the movie gave new confidence to the young actress, whose previous experience was limited to summer stock and a handful of TV appearances (The Outsiders; Star Trek: The Next Generation). ''I had grown used to feeling like an underdog, and suddenly (after winning the part) the pressure was not 'How am I going to wow these people?' but 'How am I going to live up to their expectations?''' says the Brown graduate, who is holding out for her next film role instead of jumping at TV offers. ''Maybe after this first time getting my feet wet, it will be easier the next time to feel like I belong.''