A lot of Over the Edge was conjured up during play. My gamemasters notes were remiss in that they did not demonstrate how to invent weird stuff that fit Al Amarja. Heres an example of how I came up with some of the background. With luck, it will serve as inspiration of some sort when you are a gamemaster.
There are two prominent taxi services on Al Amarja: the sleek Total Taxi and the grubby Giovannis Cabs.
Early in the campaign, a player-character looked up Taxis in the phonebook, and I had to come up with something. The important thing is to give the players a choice, preferably one thats somewhat ambiguous. That way the players can define their characters by the choices they make. So I invented two cab companies for the players to choose from: Total Taxi and Giovannis Cabs.
Naturally, the players called them both to compare. Total Taxi was smooth, sexy, elite, and expensive. The husky-voiced woman on the line offered to serve the characters total taxi needs. Giovannis was crusty, lovable, and cheap. Total Taxi offered elite, pricey service. Giovannis Cabs offered the best that the cabbies could do. Total Taxi had the gloss factor. Giovannis Cabs had the underdog thing.
In another early session, the characters were fleeing pursuers, and they called Total Taxi. A glossy black Total Taxi picked them up. For an extra fee, the taxi driver (possibly controlling the car by remote) pulled a standard maneuver from the Total Taxi evasion menu: Four-Way Scramble. The taxi pulled into a parking garage, and three other identical taxis drove in as well. The decoy taxis cut in and out until the pursuers were following the wrong taxi, and the characters got away.
Giovannis cabbies had heart, but they were outclassed. The players preferred Giovannis for routine service, and only called Total Taxi when wounded or pursued. Giovannis also provided me the repeated opportunity to improvise a crusty, lovable, possibly damaged cab driver.
If I can generalize this example into a rule of thumb, its When improvising setting, give the players options. But Im not sure that this rule applies to improvising any more than it applies to preparing a setting ahead of time.
A lot: There's an idea in indie RPG circles that almost all of the background in Over the Edge was invented through play. In fact, most of the setting was dreamed up and prepared ahead of time, in the standard world-creation style. A lot of it actually came from Robin D. Laws. A significant amount of material, however, did grow spontaneously out of play, a lot more than is typical for a campaign. In fact, the setting was designed partly to allow just such improvisation. As others have pointed out, I didnt do much to show GMs how to improvise their own material. This rant is a small step in that direction.
Taxi cards from the card game On the Edge. The Total Taxi card gets a character out of a jam. Johnny Kazoo negates Total Taxis.