Helping Eric Nylund craft "The Fall of Reach," the first HALO novel was a treat. Eric's a hell of a writer (and I actually was one of the people who more or less bullied Del Rey into using him in the first place) and a good friend, so that kind of contribution is effortless.
Working on the game itself, HALO: COMBAT EVOLVED, was a considerably less pleasant experience. The deadline pressure was enormous; my co-writer at the time, Brannon Boren, and I had just a couple days to bang out a massive amount of dialogue at the request of the game's designer, John Howard, and Bungie co-founder Jason Jones. Apparently, whoever had written the original in-mission dialogue had not done a job everyone was happy with, and they needed, essentially, a writer or writers who could work fast. We happened to be around and available, so we agreed to do it, but Bungie is VERY secretive, and, despite the fact that we operated under the same non-disclosure agreements THEY did, they can be quite mistrustful.
So, we had three days to bang out some ridiculous amount of dialogue, but they wouldn't actually let us LOOK at the game (which is why some of the dialogue doesn't make total sense; I conservatively estimate we got it about 80-90% right, which I think is an accomplishment given how blind we were flying).
THAT was no fun at all.