HELP! Copy right question(s).

I'm very confused about copyright laws and specifically laws and regulations with featuring someone's face in my film.

I'm working on a documentary film and I always get people to sign waivers for formal sit down interviews, but from time to time I'll go out on the streets of NY and interview people on the spot. I ALWAYS get people to verbally agree to "let me use the footage in my film." Is this enough? Do the same rules apply internationally?

Does anyone know of a reputable website where I can get this info? I do not have the funds to invest in a lawyer right now, but I intend to put my film in the festival circuit and as I understand, buyers won't be interested in the film if they feel that all legal angles aren't already covered.

Thank you!