A Cognitive Psychological Approach to Gameplay Emotions is a paper that Bernard Perron gave at DIGRA 2005. He adapts discussions of how viewers respond emotionally to cinema to understanding the emotions of playing computer games (specifically story-based ones.) I think he gets it right.
But inasmuch as you can make your avatar act, you have to make him take action. If not, there will be no game. Otherwise, as Iíve often stressed with regards survival horror games, it is certainly not the avatar that is meant to be scared or have emotions, but rather the gamer [19]. The avatar, incidentally, generally stays expressionless, whatever the situation. We saw that emotions depend on the gamerís appraisal of a given game situation. This individual appraisal will consequently produce subjective emotional reactions.
In many ways the mission style of certain games provides the overall motivation (you are just following orders) and it is in the achievement of the assigned task and overcoming the problems that the emotions of playing lie. Few games are sufficiently open ended that you can choose the general attitude to take to playing/living in the game-world. It would be much harder to program a game that was so open ended. By choosing the play a mission you suspend your choice of life goals in order to take pleasure in the goals set by the game. What does this say about levels of (free) will?