I just can tell about ggg-Production. The models dont get Photos in advance. You must imagine, that there are about 100 male participants. They produce about 10 scenes a day with 5 girls...
GGG makes a difference between cummers and Fuckers. The first have to bring a test about HIV and Gonorhoee/Tripper, but they can only cum on the girls. So the Health Risks are not very high. The fuckers must have a full test. Among them there are many professionells (as pornfighterlongjohn for example) and you have to cempete with them.
I only saw once, that a girl refused a cummer (because his cock wasnt clean enough).
But you can be refused by the producer John Thompson. If you are too fat, it can be, that he send you home (but its rare). The most difficult thing is to get to the girls, because many of the boys want. But the crew let only 10-15 Cummers to the girl...
John Thompson often comments your efforts in a very harsh way: (Are you too stupid to come in a Girls mouth, fat guy! Thats not enough cum! an so on).
So you got to have a big self-consciousness to participate....