Proper English would be "I watched the film yesterday" but it as also acceptable to say "I saw the film yesterday."

If I saw a film on tv, or watched it on dvd at home, I'd say " I watched it" . On the odd occasion, I might use "saw" However, if I went to the cinema, I'd say " I saw the film". In this instance (cinema) I'd never say watched. Perhaps it's a UK/USA difference, as I seem to be the opposite of Roy!