Obviously, I think that games with stories and characters need great writers. I just think a game designer or developer is destined to search in vain if he or she thinks that there's a single answer out there somewhere.
The solutions I believe are:
Don't get star-struck. Choose a writer by his/her writing ability; read his/her writing and decide what you think. Don't drop your jaw just because they're represented by a big agency.
Even if you're using a "name" writer, be aware that often films and TV episodes go through many rewrites by writers who aren't even credited.
Absolutely insist on reading a film or TV script. (It doesn't have to be a film or TV episode that was actually shot.) Is this a great writer? Can he or she make both major and minor characters unique, interesting, dimensional, and emotionally rich? Is the story imaginative and gripping? Can this writer captivate you from beginning to end?
I should mention that in Hollywood, people often write in teams. If the writing sample is from a team but you're only dealing with one of the writers, forget the writer. You have no way of knowing who wrote the best material in the script, no matter what the agent or writer claims.
Similarly, many Hollywood scripts are rewritten many times, by many different writers. If you're shown a script and you're told that the writer wrote "this draft," the sample is useless, even if it's by a famous writer. The agent and writer will protest and say the writer did a "page one rewrite," but, in truth, you'll never have the faintest idea who created or wrote some of your favorite plot twists, characters, scenes, and dialogue.
 I don't even begin to consider bringing a writer to work for me as part of my game design and writing consultancy, The Freeman Group, unless that writer has at least one TV or film script that utterly floors me with the complexity of its artistry and the excellence of its craftsmanship. For what it's worth, while it's conceivable that I might receive such a script from someone who is not yet a member of the WGA (the Writers Guild of America, which you can join only by selling film or TV scripts to major Hollywood companies), it hasn't happened yet.
Once I find a writer who is talented, skilled, imaginative, passionate, and available, and who has an upbeat and warm personality, then comes the very long process of training him or her in Emotioneering™. (Emotioneering is the vast array of techniques for creating emotion in games, illustrated in the next section of this book.) Otherwise, he or she will be of little use to me.
If the writer doesn't play games and can't think in terms of gameplay, then that writer might be helpful in certain circumstances, but the kinds of ways this person can be utilized are limited.
Even if the writer has worked on other games, still read a writing sample. I know one writer who has worked on one game after another and has done a poor job on all of them. People hire him off his resume, without stopping to read and assess his talent and skills. The proof has got to be in the pudding: Any writer must be able to prove his or her ability.
Confirm that the writer knows the medium. Make sure the screenwriter understands the difference between writing for films or TV and writing for games.
Make the screenwriter prove that he or she can execute Point 9. Can the screenwriter create emotionally complex NPC characters some of which are likable, some of which are not with very few words of dialogue?
Educate the writer to the group nature of the creative process in games. Make sure they're absolutely fine with it.
Use your gut to assess the screenwriter's motives. Does he or she actually care about games?
If you find someone who meets these criteria, they're probably going to contribute to the quality of your game, and to the buzz and profits your game generates.
I can tell you from personal experience that the task isn't easy. As I was asked to help design or write more and more games, I formed The Freeman Group, a band of professional writers who are trained in my techniques and who often work with me on game projects.
My first criterion in bringing a writer on board is that he or she has sold film or TV scripts to major Hollywood companies, and therefore is a member of the WGA (Writers Guild of America). Also, the writer needs to have writing samples that blow me away. This is a must. And he or she needs to be genuinely warm and upbeat, and comfortable working as part of a team. A strong familiarity with games is a big plus, but not nearly enough.
If a writer meets these criteria, I give him or her several writing tests to evaluate his or her skill and artistry in the kind of writing and thinking games require. Most writers, even though I only work with WGA members, fail these tests. But for those who excel, and who meet all my other criteria, I then start training them in Emotioneering and in writing for games. It's been a long haul, and I still haven't been able to fully clone myself. But I keep on trying.
My difficulty of finding writers who can assist me in doing superlative, artful, and ground-breaking game design and writing has given me great sympathy for any game publisher or developer who tries to do the same.