Casting
A porn scene staged as an audition: a new guy is interviewed on a couch and the questions turn into sex, a riff on the Hollywood casting couch.
A casting, in porn, is a scene sold as an audition: a new guy sits on a couch, answers a few questions about himself, and the "interview" turns into sex, usually with the man behind the camera or a performer who just happens to walk in. It plays on the old Hollywood cliché of the casting couch, where the part goes to whoever takes his clothes off.
A casting is a genre, not a real audition. Real studios do test new models, but the scene you watch was planned, paid and agreed in advance like any other. The fun is in pretending it wasn't.
How it works
The script hardly changes. The new guy is shy, or says he is. He talks about his job, his girlfriend or his first time with a man. The off-camera voice asks him to take his shirt off, then his jeans, then "just show us what you've got". From there it can stay a solo, or it can turn into a blowjob or a fuck with the cameraman, often filmed in POV.
The interview is half the product. Viewers like the feeling that they are seeing a man before he became a performer: the nervous laugh, the awkward answers, the "first time" that may or may not be one.
In gay porn
In gay porn the casting is closely tied to the fantasy of the straight guy who does it for money. Many casting scenes present the model as straight or curious and broke, which links them to gay for pay, straight bait and the frat and military genres. Others are simply a fresh twink or jock introduced to the audience, with the interview as foreplay.
Because the format needs nothing but a couch and a camera, it has always been a favorite of small studios and amateur producers, and now of creators who stage their own "auditions" for their fans.
The real casting couch, a producer trading roles for sex, is abuse. The porn version is a game played by adults who agreed to every step of it.