William Petersen: There has never been a show that has followed the criminalist, they follow the cops, and the cops are always chasing after lies. They chase down whoever is lying to them and try to get them to not lie. And these guys (the criminalists) use irrefutable evidence to pinpoint the truth. That's what drew me to it. I've been telling anybody who would listen that I wanted to do a series for the last 10 years. But I wouldn't do it if I was just another cop pushing bad guys up against the wall.