A game or movie environment can be totally different from the real world we inhabit, but still be believeable to us. How?
Good art direction that ties the made up environment together consistently. The imaginative world follows its own logic and as long as the world follows that logic, we’ll believe it and understand that this universe works that way, and we can immerse ourself in it without questioning believeabilty. But when art direction slips and the universe breaks its own rules, the universe looses credibility and immersiveness, as we no longer believe in that world.
That is why art direction is very important. Art direction is the visual glue in video games. When you put everything; the environment, the lightning, the characters, the props, together, good art direction is what makes the simulated world seem logical within itself. Mood, visual style, viewpoint, and colour palette are all elements of art direction that need to come together to create great visuals.
Art direction in film and art direction in games is very different, I think. The biggest, most important difference between games and movies is interactivity. The creation of an interactive world the player will explore and be a part of and hopefully feel engaged and immersed in is very different from showing a viewer a movie; the movie will always be the same no matter who views it, but different players may wish to play the game differently. You also need to tell the player what to do and direct the player, while the movie can simply go on whetever the viewer understands it or not.
To be a good art director I believe you have to be very good at drawing, and very fast. Drawing is the cheapest and quickest way to generate, visualise and crank trough ideas, giving you the opportunity to nail what you need to nail in a non-time consuming way, saving you time and money when you start the time consuming processes of modelling your world or characters or filming your clips.
I think you’ll need a very good grasp of colour theory, lightning and perspective. These are essential tools to setting the mood and telling a story with pictures as effectively as possible.
And I believe you need to be curious about the world and always eager to learn about new things. You'll need to conduct a lot of research; read, listen and see.
You’ll also need to not be precious about your ideas; killing your darlings, things you really liked, might be necessary to obtain the good end product. You also need to be able to tell your bad ideas from your good ones, and more importantly, be able to crank out all of them so that you even have your good ideas in solid form.