Programming for Interactivity vs. Passive Viewing
The next difference to keep in mind is the amount of programming that goes into a video game's animation, interactivity, and rendering. Because a movie is meant to be viewed but not interacted with, the programming inherent is only oriented towards producing visible results without any input from a user; the models don't need to be able to react to stimuli appropriately, because they're not reacting to anything at all. In video games, every action is controlled by the user; motion sequences must be programmed in as a response to individual buttons or combinations of buttons; then as a result objects or beings in the environment have to be programmed to be "sensitive" to the actions of the user-controlled models, to enact their own programmed motion sequences at the appropriate moments.
For example: when playing a combat-intensive game, an enemy's model has to be programmed to enact the "attack" motion sequences when within a certain range of your character's model, on top of being programmed to gravitate towards your character's location. Your character's model is programmed to move in certain ways and lower the character's statistics if the enemy model comes in contact with it in certain ways, causing "damage"; however, other than taking damage and possibly dying, your character won't respond until you press the right buttons to attack, defend, or retreat. Should you attack, this initiates another motion sequence, and when your weapon or other method of combat comes in contact with the enemy character's model, it in turn reacts by taking damage and lowering its vital statistics, or even possibly dodging before resuming its programmed attack behavior.
AI vs. Scripted Motion
Various "artificial intelligence" (AI) engines have been developed in many gaming environments to help control in-game character behavior, and make it "smarter"; in their own way, video game models are more "alive" than movie models, because they react to stimuli in certain fashions and are even capable of "learning" and storing past behavior in the game's memory; movie models, in contrast, have no need to do that, because they're only enacting a script precisely, controlled by animators as little more than puppets.
Environment Interactivity
The level of interactivity doesn't stop with character models only, however. It also extends to objects in the environment, which can trigger certain actions, be picked up and moved around, or be examined, moved, or thrown. The information for these items has to be stored and accessible in response to your character's actions, so that the objects respond in the proper fashion. Some games can have thousands of individual items, each with their own set of individual data, each fully interactive.
So you see, while interactivity vs. passivity is the main thing that separates CGI video games from CGI movies, that simple description just barely touches upon the surface of what those differences mean. There are infinitely more nuances and applications that separate the two genres, but they're united by a single common cause: the desire to captivate the user or viewer and involve them, in one way or another, in the story that unfolds.
