I watched most of the film, but not all of it.
You are talking about ONE film. You didn't watch all of it. You stopped watching this film before it ended.
I watched most of the films.
Here you are talking about several films. Maybe you stayed somewhere for the weekend where there were a lot of different films on DVD. You watched a number of these films, but you didn't watch all of them. Maybe there were 10 films and you watched 8 of them.