I remember a particular TV film where the protagonist becomes obsessed with his inner world of high fantasy and this becomes an important cause of his actions, but there were no other elements of fantasy in the story.
(Edited subsequently)
I must add that the actions of the protagonist are influenced by his inner world of fantasy, not only because it is very important to him, but because the protagonist believes that this fantasy world is real.
So it's not just the case of an RPG player obsessed with his hobby, but a situation similar to Don Quixote's: he sees giants instead of mills.