A Hollywood Cinderella story: Once upon a time (last year) former stand-up comic Steve Hytner, 31, toiled tirelessly in small parts (like in Ski Patrol, a movie dubbed ''Police Academy on skis''). Then he took an unassuming job as a ''reader,'' feeding lines to actors auditioning for a new Hollywood Pictures' movie. Hytner caught the attention of the film's director, Jerry Rees, who was so impressed that he arranged an audition for Hytner with screenwriter Neil Simon. Poof! Hytner snagged the plum role of George, one of Alec Baldwin's four good pals in The Marrying Man. ''It was the most exciting point of my career,'' says Hytner. ''I called my mom and all she could say was, 'Oh! Oh! Oh!''' Despite rumors of feuding on the set, Hytner says he and his male costars ''became a really tight-knit group.'' The five actors (Hytner, Baldwin, Paul Reiser, Fisher Stevens, and Peter Dobson) even took a four-day weekend together in Las Vegas where, Hytner says, he ''beat Alec pretty good on the tennis court.'' Baldwin didn't take it too hard: He later sent Hytner a $300 racket as a gift. Now, Hytner is busy starring in the new NBC television series The 100 Lives of Black Jack Savage and frequently auditions for movie roles. It seems like he'll have to wait to break in that racket.