On Oscar night, 150 volunteers will wind up with some of the best seats in the house. These are the seat fillers (including: Joan Wright, Vicki Davis, C.W. West, and Sandy Christmas), the people who will rush to take the places of presenters and winners called on stage, so that viewers won't see gaps in the audience. They may even get to hobnob with their star seatmates. Recalls Davis, a clerk in ABC-TV's Commercial Integration Department and a four-year Oscar vet: ''When Geena Davis [no relation] won Best Supporting Actress in 1989, I was called down to sit next to Jeff Goldblum, Meryl Streep, and Cher.'' Once there, she obeyed the seat filler's golden rule don't speak unless spoken to but Goldblum initiated a ''very friendly'' chat. Two hours before show time, the seat fillers will meet with their boss, ABC's manager of guest services, Joe DiSante, to review technique. ''I show them how to get in and out quickly and which way to face,'' he says. ''It's rude for people to speed through with their butts in everyone's faces.''