To be honest, if you're shooting anything that is going to be used commercially on a location, say, downtown Anytown, USA you have to have a permit to film there.
If you are going to use anybodies image on a commercial presentation you have to have a written agreement signed by that person with a copy of their identification to be extra sure.
This doesn't protect them, it protects you. If for some reason you have a fantastic effect filmed and say some tv company or magic company wants to sell an effect with that footage, you need a signed release to allow you to use their faces. If not, you can always mosaic out their face. But that sucks.
I've never had a problem with it, usually I tell them, "Hey, I'm shooting a documentary on magic, would you like to be a part of it?" then toss out a standard release form (you can find some good ones online). Nothing uber long, just something quick and to the point.
I've never had anyone say no.
Now, on the other hand if you're shady and just use it anyways and someone finds out later, you could be sucking when someone finds out and comes with a lawyer. Remember, the new American dream is to get rich suing someone.
