And of course, it all depends if Phoenix actually has a lawyer on retainer. Some don't. As I recall, Crispin Glover put on a memorably freaky Letterman show. No one said squat about his weirdness because the guy was weird.
If Phoenix doesn't have a lawyer, that still doesn't invalidate Dana Stevens' reasoning that Stiller wouldn't have risked the libel suit and appearance of gross insensitivity by making fun of his mental illness/addiction/whatever.