I think Kevin has summed it up quite well, but one more thing to add. Charters on the C&TS tend to be two or even three day affairs and marking that many locations would be a major challenge. On the Lerro/Farrail 2012 five day event, I think I counted 135 runbys. Since some of those were two per location, that still means on the order of 70 discrete locations. Also, when two runbys are done per location, people often change positions for the second runby.
My experience has mostly been with operating 40 person charters and people have gotten along well and moved when asked to allow others to get their shot. The 65 person Friends charter this last fall was interesting, since quite a number of the riders had not done a photo charter before and needed quite a bit more guiding. Several people shouted at non riders to move out of the way at the MP 305 curve and this was indicative of the lack of experience. Also, at least a couple of the people being yelled at were Friends members who were there for a work session. Walking up and talking to folks is the way to get cooperation. Future pre charter briefings will cover this situation!
I was on a charter at Steam Town many years ago when someone in the photo line yelled at a guy to "get out of the shot." The guy very politely said that he would be glad to, but that since he was the brakeman and was walking the train for a brake test, the train wouldn't be going anywhere as a result. You could hear a pin drop in the photo line!
Michael Allen