One thing that really helps is to write a lot MORE story than you actually put into the game. Write a background story for each and every character, even if the character is not a major one.

For insight on what one of the BEST game designers did, you should read the Deus Ex Bible. Warren Spector is a genius when it comes to game design, and you will see that there was a lot of detail put into the design of Deus Ex. If you have played the game, you will notice that some of these details weren''t actually in the game. But if you read them, they make sense.

For example, I did not realize that Tracer Tong sold augmentations on the black market in China. But after reading through the DX Bible and finding that, it makes perfect sense. Was the story any less interesting because this was left out of the final game? No. Was the character more rich because all of the writing had a well-developed character as the basis? Certainly.

--TheMuuj

I would imagine a detailed written game design also helps to guarantee a consistency in the characters. Otherwise, a gruff character you spoke to at the beginning of the story could be strangely passive in the third act.

Or that same character could act as though he has an entirely different background... an excuse to have the information you need to get you to the next quest.