My experiences have all been with Treasure Island Media. They take a documentary approach to their material. They find guys who have complementary or similar fantasies through auditions where participants are asked about their favorite sex acts, their hottest fantasies, their likes and dislikes in partners and activities, any restrictions, their HIV status and other STDs. Then they are photographed or filmed (for us tops I think it is to make sure we are not camera shy and have great erections regardless of the situation). I know that the bottoms are traditionally allowed to choose the tops they want ahead of time from the audition photos or vids, but I don't ever recall being shown the bottoms beforehand (although I was told when it was going to be Dawson again). Days before the shoot, participants are told specifically not to take drugs and to not have sex for (as I recall) 18 hours before they arrive, and they are also told the theme of the sex and given suggestions for what to wear or bring. The day of the shoot, everyone signs releases and waits for the start of the action. They've already lined up a great play space and staffed with with camera and light crews and use a mixture of fixed and hand-held cameras. They keep the tops and bottoms separate until they are ready to start shooting and then open the doors for whatever happens next to be recorded. Ample lube is around, as were poppers. No scripting, blocking, dialogue, etc. It is much more like any sex party, except there are a lot more hot sexy guys AND you know that everyone there is really into the sex that is going on. They capture scores of hours of film (there are rarely any breaks) and I know they have a lot of great film to choose form when they compile the finished product.
That is NOT what I understand most porn shoots are like. So hopefully someone else will share their experience of a traditional porn shoot.