A number of years ago, I had the opportunity to film, but pretty much chickened out (was more afraid of what I feared was the amount of pain involved than being on camera). I wanted to ask if it would be possible to film a Spitting scene, but didn't because 1) I didn't know if it would be deemed BDSM activity and 2) I didn't want to appear out of place.

Fast forward a few years (and continuing this day), damn if there aren't a ton of spitting videos.

Lesson learned here, don't fear asking questions or expressing your interests (just be sure to be respectful of the other party's limits).