You do realize when you're doing this kind of show, somebody's going to bite it. When I talked to the casting director, he said, ''I can't tell you exactly what's going to happen, but it's pretty gruesome.'' [Monahan's Good Samaritan jogger is killed by Edward James Olmos' and Colin Hanks' characters in a ritualistic murder in the Oct. 16 episode.] I was sitting in the head cast for, like, six hours while they were making the mold, and the guys who were doing the prosthetics were leaking out little bits and pieces of what was going to happen to me. They were talking about painted horses, riding through town, and decapitation. Needless to say, my mother didn't chime in for this particular episode. I've died before on screen. I've been shot, strangled, committed suicide but that last image of the show, even I had that moment of ''Oh, that is pretty disturbing.'' As for the head, I don't know if I'd want it as a souvenir. What if you go for that 3 a.m. pee and suddenly you run into it? Some things are best left in the prop department.