How important is the cover or poster of a movie?
Very. First impressions count, and the poster/cover is usually the first impression.
I do remember when I was younger, I'd look at a movie and be looking for reasons to go watch a movie. Meaning, if the poster, review, description had an element that I was interested in, I'd go and see that movie.
Now I'm a little older, I look more for things that cause me to say no, I'm not watching this movie. Eg. common elements of a bad movie. It could also be that we're in different times. I'm finding that there are more movies now than ever before. Filtering down the movies to what I'd enjoy is what I do.
Have you ever decided not to watch or buy a film because of the cover, or subconsciously influenced because of a bad cover?
See above. A bad cover is usually the first impression of a bad movie. There are so many elements that go into a good movie. Bad sound, bad acting, bad story, bad pacing, boring characters, bad presentation, bad lighting and so on. They are all just elements that all go together to make a great movie. The cover is usually one of the elements that people polish up so much (and the trailer). It's really rare to find a bad cover and a great movie. So yes. If I see a bad cover, I'll wait for a great review from a source that I trust or I just don't see the movie.
Do you think it plays an important role in selling an indie film?
Yes. Just plain and simply yes. The cover defines the market it is aimed for.