Depends on the scene, honestly. I've been on some modeling gigs where the thoughts are all about what I have to get done after, like grocery shopping. There are some days where I just worry about how I look, and others where I'm talking about Doctor Who with a photographer the entire time. I once had a shoot that was really a cover for a massive "Wolverine vs Batman" debate. On porn sets it's a little different. A lot of what goes through my head is stuff like "I hope the director's getting the angle they wanted" and "What position did they want us to do next?" Sometimes I get lucky, like when I shoot for Kink.com, and I can really get into the sex. Then my head fills with two revolving thoughts. "Holy shit I'm cumming again!" and "This is going to look so hot later!"