Some celebrities seemed to spend more time on the witness stand than on the silver screen in 1989. There was, of course, Zsa Zsa Gabor’s trial for the slap heard ‘round the world. Actor Don Johnson was called to testify in the case of an alleged burglar he apprehended Miami Vice-style in his room at the Bel-Air Hotel. Most celebrity court cases, however, were of a far more serious nature. KNBC-TV anchorwoman Kelly Lange told a hushed courtroom of a stream of letters she received from an obsessed fan, including one in which the fan wrote that he was going to “blow (her) head off.” Warren Hudson, 57, was convicted and faces up to three years in prison when he is sentenced in January. Actor Michael J. Fox, meanwhile, testified that he was forced to change his life style after receiving nearly 6,000 threatening letters from a Westlake Village shipping clerk. The defendant, Tina Marie Ledbetter, was sentenced to three years probation. TV actress Rebecca Schaeffer will never get the chance to testify in the case of an obsessed fan who stalked her. The 21-year-old co-star of “My Sister Sam” was shot down in cold blood at the door of her apartment. Robert Bardo, a 19-year-old Tucson resident, will stand trial. On the white-collar side, Beverly Hills “junk bond” wizard Michael Milken was indicted on federal racketeering, insider trading and security fraud counts. Milken, who was paid more than $500 million in 1987 by his employer, Drexel Burnham Lambert, now faces a maximum prison sentence of 300 years. In Civil Court, actor Timothy Hutton won a $9.75-million judgment against MGM for fraud in the cancellation of a 1983 film project. And Rock Hudson’s one-time lover, Marc Christian, was awarded $5.5 million from the actor’s estate on the grounds that Hudson, who starred in “Pillow Talk,” failed to disclose to Christian that he was suffering from AIDS.