What do Kevin Costner, Jane Fonda, and Peter Bogdanovich remember most about their first time? ''Costner remembers Ben-Hur not seeing the movie but asking his mother what the title meant. For both Fonda and Bogdanovich, it was Dumbo,'' says film documentarian Chuck Workman. For ''The First Time,'' a five- minute segment featured on this week's Academy Awards telecast, Workman asked 50 stars to share their earliest and fondest movie memories. ''The whole theme of the Oscars is the history of the movies, so this is just kind of a nostalgic moment in the middle of it,'' explains the filmmaker, whose fast- paced montage, ''100 Years at the Movies,'' opened last year's awards. Workman's latest feature-length project, Superstar: The Life and Times of Andy Warhol, is currently opening across the country, and it follows the same zippy pace. ''Warhol did one shot in a hundred minutes,'' says the director, referring to such leisurely Warhol films as the nearly still-frame Sleep and Eat, ''and I do a hundred shots in one minute.''