If you mean at the movie theater....(thus the, "do you just leave"), I would go and watch another movie. They charge an arm and a leg, I am not leaving without watching a good movie. ;p
Only once have I left a theater for that reason. But when I'm watching a download or something, I tend to skip ahead when it gets dull.
No but if it starts out sucking and continues to suck then I will leave.
Did that once, never again - I'd leave.
Depends how bad, who I'm with and if they like it
I simply fall asleep.