Many times I have asked someone "Have you seen [insert film title here]?"
And they say something to the effect of "I've only seen parts of it on TV as I was listening to my iPod, eating food, and walking in and out of the room."
Then I'll say "You should watch the whole thing. It's pretty good."
And they'll say "Nah. I basically already know what it's all about. It's not that good. I didn't like it."
This entertaining video shows why that argument doesn't work:
Moral of the story: If you haven't seen the film, you may not know the whole story. . .and people will laugh at you.