Gumshoe is going to let players create a story that's like the film noir of the 1930s, 40s, and 50s, or like the stories of Dashiell Hammett or James Ellroy. You've got hardbitten detectives walking down mean streets, where everybody wants something and everybody's got something to hide.
Right now, I see the game taking the following form:
(1) The players get together and specify a locale and a time period, like 1950s LA or Depression-era New York. It may or may not be necessary at this point to go down another level of specificity: okay, it's 1950s LA, but is the story centered around a movie studio or a precinct house? At some point, location cards get selected and laid out; the location in which a scene gets played out will have some effect on conflict resolution. The selected locale will have an effect on what location cards are "playable."
(2) Players come up with potential cases framed as situations or facts of the the case, writing them down on index cards. "My wife is missing," "There's a man who owes me money," "I want to get away from my violent pimp." Players then vote on which case the game will be about. I envision a kind of bidding, in which players distribute six points among three cases (i.e., 3, 2, and 1, or 2, 2, 2, or even 4, 1, 1), not including their own. The case with the most points is the one the game is about.
(3) Now roles need to be assigned to players. One will be the Client, who plays a large part in defining the initial situation of the game. Another will be the Gumshoe, who is most active really only at the end; the rest of the time, he's just going around enabling other characters to define themselves, and position themselves with respect to other characters. Other players take the roles of various antagonists and supporting characters in the story.
It's important that there be a chance for any other player to be the Client or the Gumshoe; the way to do this I think involves another round of bidding in which you allocate the points you got between Client, Gumshoe, and other roles. This gives the player who got more votes an advantage, which may require outlawing 2, 2, 2 as a distibution of votes, since one is motivated to do that by the strategic concern not to disadvantage yourself by giving someone more votes than you're likely to get.
(4) Once roles are assigned, the Client starts things rolling by framing a scene with the Gumshoe in which the basics of the case are outlined. Then other players can specify their characters, including their Secrets and their Desires (or maybe the client parcels those out based on another round of writing).
(5) Other players get to frame scenes wherein they encounter the Gumshoe. Statements are advanced, subjected to doubt and verification, or revealed to be lies by the mechanics of game resolution.
(6) The Gumshoe gets to frame a final scene in which the central conflict or mystery of the game is resolved.
That's what I've got so far. Stay tuned!
Bill White's roleplaying game design blog, with emphasis on narrativist or story-heavy games.