Over the past couple of decades, a clearance culture has built up, so that everyone is scared shitless that someone is going to sue their asses off if they include some copyrighted material in their movie without paying for it.
For example, you are making a documentary, and one of your characters is a kid, and while you are shooting with them, they put on their favorite cartoon. And maybe this cartoon is really important to them, and it's a scene you want to include in your movie. But you don't. Because you are just a young filmmaker with no budget, and you can't pay Time/Warner five thousand dollars to use a clip from their show and you're worried they'll sue the hell out of you if this ever gets in to festivals.
STOP. Use the clip. This is an example of Fair Use.

