Eurogamer has an interview with Irrational Games' Level Designer, Dean Tate. You might know of Dean Tate as the recipient of the Australian Game Developers Conference 2004 Award for Best Level Design (Tribes: Vengeance). Dean talks to Eurogamer on the expansion pack for SWAT 4: The Stetchkov Syndicate...
Dean Tate: When planning a level I like to look at it from two distinct points of view, the game mechanics and the story.The first thing we did when starting the expansion was to sit down and brainstorm lots of cool, interesting things for the player to do in the levels. We wanted to throw interesting twists at existing locations in the real world: take something the player knows, then turn it on its head! With this being an expansion we thought it was important to take all the cool gameplay mechanics of the original and find new ways for the player to experience them. We also worked to strengthen that feeling of "being there" by providing each level with its own story fiction.