Most film and TV programs (unless very old) are protected by copyright. If film is not protected as an entity, older films are often protected as a collection of stills - so there will still be copyright issues to consider.
So while use in research, study and teaching may be ok, other uses may require permission. Film and TV programs should always have full citation details (see moral rights).
Please check the Monash Copyright information site for more detailed information about use of YouTube and other film and TV material.