They actually speak in those things???
The occasional and presumably accidental “intrusion” crew members into certain scenes; reflected in windows, when the stage lights are obviously too close to the “actors” and glaring off ass, thighs, etc. I remember once a four girl scene in which you could not only see a crew member (or, maybe an actor) almost out of frame, holding up a girl’s leg. And then you could hear him on the soundtrack doing some hysterical dirty talking.
From Paul Newman, of course. 
I think it may be to “re-harden” it… remember, shooting of a given porn scene takes hours, so the little guy might need to be smacked about a bit to keep him awake, as it were.
As for the smack on the ass while doing someone up the butt (I see this all the time in gay porn), I read somewhere that it causes the fuckee’s sphincter to tighten momentarily, which apparently is pleasurable to the fucker.
Not that you’d ever catch me doing that. Ta-a-a-cky.
- s.e.
It’s like a White Zombie song!
Moreso in internet porn than in movies, but:
Foot fetish 
What’s the big turn-on supposed to be? I don’t find pictures of people’s feet to be even the least bit interesting, much less erotic.
Oh, I must repeat spitting.
The most recent porn flick we watched included several scenes where guys were spitting on the girl, I presume for lube.
However, it was done without the least bit of tact. I think major league baseball players are more discreet than these weirdos.