You're sitting there enjoying yourself and watching some porn and trying to imagine yourself in the scene. If you are honest with yourself, you know you must look pretty silly as it is and no one's around so why not take a few steps to make it more realistic?
I often find myself jerking my cock or bucking my hips to the same rhythm as the people fucking on the screen. Sometimes I might get myself into the same position as them. I may, for example, get on my knees because the couple I'm watching are doing it doggy style. Or I may suck on a finger as she sucks his cock.
How embarrassing would it be to get caught? How much more so while mimicking the scene? I wonder how many others do this?