You need people’s written permission to use their likeness (that is, their face) in your film – if they are recognizable on screen. However, if you are shooting a crowd scene – at the Rose Bowl, for example – and if you do not focus on any one person or show their image for more than a second or two, then you do not need their permission.
It may also help to post signs around your location like the following:
“ATTENTION!” Today there will be motion picture, video and still camera crews from CalArts School of Film/Video photographing areas marked by these signs. Their work may become part of a movie or video distributed in any and all media. If you do not want to be photographed and thereby allow for your likeness and photograph to become part of the film or video, please do not enter the area bound by these signs. By your presence in such areas you are consenting to the use of your likeness for the above stated purpose.”
A sign like this does not absolve you from the responsibility to get the proper releases, but it may prevent some unpleasantness with the citizenry.