If I find out that it's not a good play or musical I'll usually skip it no matter who is in it.
There are some shows that I am more likely to go to because of an actor/actress, like going to see Dustin Hoffman in "Death of A Salesman" but I also went to see it with Brian Dennehy so for me it was more about seeing the play and the bonus were those two wonderful performances.
After all "the play's the thing".
"Smart! And into all those exotic mystiques -- The Kama Sutra and Chinese techniques. I hear she knows more than seventy-five. Call me tomorrow if you're still alive!"