I am a full time wedding photographer, but my wife used to work in TV and commercial production and we have lots of friends in the industry so I will occasionally do BTS/promo stills for fairly big budget film/tv/commercial productions.

• I have always done full day day-rate BTS. Production days are very long (usually 12hrs or much more). People have always been cool and it is generally more laid back then weddings, but more active then standard product or fashion type work in my experience. I have also felt the need to pitch in with the production crew if I am not taking photos which means some heavy work. I once had to trudge for 12hrs straight around in the gator infested Florida Everglades in hip waders...
• I have always been hired by the production company whether tv, film, or commercial. So generally the EP giving some direction with some additional direction from the DP, AD, Director or even the PR person.
• Sometimes a day or sometimes months lead time. But there isnt really any prep needed other than a quick pre-pro briefing from the producer.

Bonus: Went to a BTS job once and I ended up as the main subject of a "reality tv" episode because of someone backing out. Was kind of fun and they let me name drop my business name on the show and now every once in a while someone will call me out for seeing me on tv.