IF YOU DON'T HAVE ANYTHING nice to say, don't say anything at all. If everything you have to say is nice, you'll sound like Johnny Depp talking about Marlon Brando. "He's the most generous actor I've ever worked with and one of the most generous people I've ever known, in terms of advice or feelings or whatever," says Depp, who bonded with Brando on the set of "Don Juan DeMarco," coming in April. In the film, Brando plays a psychiatrist assigned to treat Depp's character, a young man who claims to be legendary ]over Don Juan. But the two had plenty to discuss off camera as well. "It's like meeting an older guy in a bar," says Depp. " You just sit there and talk to him for hours and hours and hours, with his fascinating stories and fascinating thoughts." Not only is Brando "magically brilliant" in Depp's estimation, he's "one of the funniest guys I've ever met in my life. He could be the funniest." If Marlon ever needs a godson, it appears he's got his man.