I went to a panel of authors at Bouchercon called "Mysteries and the Movies." Robin Cook, Charlaine Harris, Joseph Finder, and Chelsea Cain talked about their experiences turning their books into film or TV shows. A couple of them shared the desire to "do a Hitchcock" in the films of their books--in other words, make a brief appearance in the film. Robin Cook mentioned he got the idea when they were filming Coma. The director agreed he could sit in a wheelchair in the hallway of the hospital when Michael Douglas figures out what's going on and starts running through the hall. "But don't say anything, and don't look at the camera."
Joseph Finder, who's an excellent thriller writer, by the way, wanted to be in the movie recently made from one of his books--and I'm sorry, I can't recall the title. There was a courtroom scene, and they agreed to put him on the jury. It was a military trial, though, so he had to get a short haircut.
After a few days, they thought he had "presence." So the director made him "assistant prosecutor," a part that hadn't been in the script. He was supposed to sit at the prosecution table and glare at Morgan Freeman, the defense attorney. "But don't tell anyone who you are," the director said. "I don't want anyone to know the author is here." The prosecutor started talking to him, and asked him if he understood what the story was about, what was at stake in the scene, etc. "You really should know what's going on in order to play your role," he said. He proceeded to tell him the story of the whole book until Finder finally whispered, "Look I know what's going on, I'm the author."
The prosecutor was all excited and yelled, "Hey Morgan, hey everybody! Guess what? This is the author."