A person's opinion of a film is supported by things that they see, hear, or otherwise experience in that particular movie.

For example, the opinion "Character A's character development was good because of X, Y, Z" is supported by events X, Y, Z that objectively happened in the film. The impact and importance of those events is entirely subjective, but the foundation of a person's opinion on a particular piece of art is often built on facts. I think that is the reason for OP's qualifying statement.