Bill Irwin is a clown. But please, don't think of that as a slam. In fact, it's the highest praise we can lay on the elastic-bodied comic, who began his career in the Pickle Family Circus back in the '70s, but by the early '80s had become a wonderfully weird presence in the New York theater world. Movies and TV would soon call for Irwin's modern-day Buster Keaton shenanigans, but would rarely utilize the full arsenal of his talents. Happily though, he's currently prepping to star in Broadway's Who's Afraid of Virgin Wolfe and gets a loving tribute tonight on PBS's Great Performances (check TV Guide listings). "Bill Irwin, Clown Prince" is a behind-the-scenes look at Irwin and his craft that proves what we've always suspected: Being a clown ain't always a load of laughs.