If you are going to be catching people that are not part of your project in your frame it is best to always let them know that you are filming and make sure they agree to be in the shot. If possible it is usually best to get a photo release, but in most cases verbal consent should be sufficient. If you project is controversial in some way they may regret their agreement, so in this case you need to get it on paper. If you are going to focus in on someone doing some type of activity you absolutely must get clearance, otherwise you leave yourself open for being legally labeled a video stalker.