I haven't had time to read all the posts so I hope I am not duplicating something that has already been said. Having run 13 years of photo trains for the DSNG, I've learned that communication goes a long way and anyone in a fluorescent vest has greater authority.
COMMUNICATION: As has been said, many people just simply don't know how to behave on a photo line. Getting in someone's shot is not deliberate. Whether on private property or public property, having someone looking official who will take the lead to guide folks is very helpful. MOST people will listen. Explain WHY/HOW they are in someone's line of sight. Ask/Remind folks to be quiet for those shooting with sound.
Just my two cents on this discussion.
Yvonne