^^
plus sometimes, they actually have nothing to do with the film even being made. Executive Producer is pretty much just a "thanks" credit.......
Vince McMahon recieved an executive producer credit on the scorpion king (and probably all Rock's other films) simply for letting The Rock do the movie.......Rock was under contract to McMahon.
Executive producer is a role in the entertainment industry that is difficult to define clearly. Executive producers vary in involvement, responsibility, and power. Some executive producers have hands-on control over every aspect of production. Executive producers are commonly the main investor or financier of a project.
There's also a practice of naming celebrities executive producers of a project in order to help gain approval. This is more common in film, for example in Donnie Darko, actress Drew Barrymore was given an executive producer credit because she contributed money to the film and helped raise its awareness.
Thanks to online encyclopedia. hope some of that helped, basically sums it up in part.