Wednesday’s Geffen Playhouse premiere of “The Exorcist” was an imaginative but pared-down adaptation starring Brooke Shields, Richard Chamberlain, Harry Groener and petite twentysomething Emily Yetter in the Linda Blair role.Theatergoers in Westwood buzzed about creative consultant Ray Teller’s impressive levitation effect as well as the spooky sound design. Fellow magician Ricky Jay chatted with Teller, while fans besieged William Peter Blatty, author of the original novel. Blatty and John Pielmeier, who adapted the novel for the stage, explained the decision to create a more streamlined version, sans headspinning. “Everyone knows how it comes out, it’s not a mystery anymore,” said Blatty. “We wanted the audience to use its imagination,” explained Pielmeier.