Filmmakers need "releases." (A release is a binding contract in which someone foregoes a right to sue you for specific types of claims. For example, without a written release, if you reproduce images of an individual, that person might have their attorney send you a "cease and desist letter" telling you to stop or they might bring you into court and sue you for various violations of personal rights.)
iRelease, Easy Release and Release Pro are all $9.99 iPhone apps specifically designed for photographers and film and TV producers.
All these apps allow a producer to quickly get someone's legal permission to be in their project - or to use a location or a photograph.
In addition to getting a signature, all these apps apparently (I haven't used any of them yet) integrate a photograph of the signee into the release - eliminating the chore of matching pictures to the correct release once back at the production office.