Theoretically, you might think a movie will be bad, but upon watching it, think it's good. And you might think a movie will be good but it turns out to be bad. The latter scenario happens all the time, and certainly part of that is due to the fact that you actually go and see the movie when you think it will be good, and only then can your expectations be dashed. If you think it will be bad, few will go through the trouble of watching it to confirm the fact.

But a movie is not a black box. You get some idea about what's in the movie from other channels--previews, reviews, friends, knowledge of the writers, or directors, or actors, etc. So in my experience when you think a movie will be bad, you are almost certainly correct. Unfortunately the inverse is not true... at least not for me.