Writing for Games
When writing a video game script you have to remember that your primary audience is not the game player but the game developer and what the developer needs is a complete picture of what your game is about. This means that you are not just writing a story but you are creating a world complete with a tone, sounds, characters, story, plot, and subplots.
Guidelines and tips for writing a video game script
Today's video games are based in complex worlds and they tell stories. No longer does a player simply advance through repetitive screens slaying goblins and ghouls in a quest toward the goal. A player now expects to progress through a world where there is a rich history and a plethora of decisions to be made. This adds to the complexity of writing a video game script and it also adds to the richness of the creativity involved.

Here is an overview of what you need to write and why.
Write an Executive overview of the story in prose
This is the most important part of your game script and this is what will sink or float your script. This overview has to tell a compelling and unique story and it should tell the complete story from the opening scene of the game through the major steps all the way to the completion of the game. An overview like this can be almost any size and it would be very easy for this to be ten written pages or more. Remember that today's video games are very complex and the stories can be very complex. This overview is also the most important part of the script. You would shop this to game developers to see if they are interested in developing it into a game.

Create character descriptions and bios for all the major characters in the game.
Writing the actual storyboard script

Here is an example
Scene 1:
Location : A dark cathedral with stained glass windows. An NPC is kneeling before a stone casket in the center of the main room
Music : background music of an organ playing introduces the scene but subsides
Characters : Main player, NPC named Thomas
Player Goal: Discover the location of the underground lair
Action: Player must initiate discussion with Thomas, upon first contact we activate cut scene (1) where Thomas morphs into a were-creature and summons his were-minions. Main character must battle the were-minions then re-initiate discussion with Thomas.
Flowchart: No decisions made at this point: If battle is completed Thomas reveals the entrance to the underground lair and player advances to that level. If player is defeated in battle revert to death cut scene (11) and move to try again screen.
Notes: Player is locked in the cathedral and there is no exit. The only viable way out is to initiate contact with Thomas. Random were-creatures can be activated if player explores cathedral before talking with NPC.