Lead narrative designer Mary DeMarle said:

“I always say it takes a combination of talent, passion, persistence, and luck. First, you have to master the craft of writing. You need to understand what makes a good story; how to develop strong, compelling characters; and how to expose characters through what they say and what they don’t say. You also need to know what it means to explore a story’s theme and why theme is critical to creating a good story. Then, if you want to write for games, you need to know what a game needs, and the constraints those needs place on good story-telling.

After this, it really comes down to who you know. Sadly, writing jobs are very hard to come by in the games industry. While a game may need 50 programmers or artists working on it, they usually only need one or two writers. (Games which require more, like Deus Ex: Human Revolution, are very rare when you look at the industry as a whole.) Because of this, persistence, luck, and making industry contacts becomes very important in job hunting. Almost every game writer I know has told me that MOST of their writing gigs came their way because someone recommended them to a producer, narrative lead, or game designer. (Myself, included!)”