Are the Contestants Actually Random?
Here's where it gets a bit tricky. If you've watched the end credits of the show closely, you've seen the line, "Some contestants have been pre-screened prior to their appearance on the show." This opens up a whole can of worms that leads to all kinds of rumors and assumptions.
What we know is that some contestants are indeed random, and picked up off the streets of New York. Not all of these contestants make it on the show, even if they play a complete game, however. Keeping in mind that the goal of any television program is to entertain, there are bound to be some contestants that just don't deliver.
Other contestants are indeed screened and/or recruited. There have been reports of a variety of different methods for this process some people have said that they were approached on the street and asked if they'd be interested in appearing on a game show. Others have attended auditions and played mock trivia games.
The key thing to understand though is that none of these contestants, pre-screened or otherwise, know that they are about to play Cash Cab. Those who have auditioned or been recruited are told that they have made the cut, and that a taxi is coming to pick them up and take them to the filming location. Of course, when that cab arrives, it's the Cash Cab.