Sentiment Analysis of Movie Reviews using Python
Given that exact case, I would assume that you are getting negative decision due to names, mentioned in the review (in your training dataset actors were more often met in negative reviews). You should, probably, remove all non-relevant words from reviews, and that includes not only stop words, but all person names (since they are less of a sentiment marker except, maybe, justin bieber, who is a really negative marker to anything :) ).