by Liz Smith
AT THE four-hour "Hamlet" premiere the other night in Lincoln Center, the big stars in person were Kenneth Branagh, Derek Jacobi, Alan Alda and Kathleen Turner. (The last two weren't in the movie; just the audience.) But the celeb who had everybody's neck craning was a dashing, sexy Keanu Reeves. The reluctant screen idol (remember, he turned down the "Speed" sequel!) was there with a tall red-headed beauty. All I could find out was her exotic first name - Acacia.