At the end of the credits of every movie, I read the message saying, "The events depicted in this movie are fictitious. Any similarity to any person living or dead is merely coincidental." However, in a number of interviews with screenwriters and on several DVD commentary tracks I hear a writer talk about a real person or experience that he or she used in the script. Why do you they get to put that message at the end?
This disclaimer is there strictly to help protect the studio and filmmakers in case someone comes after them, claiming libel or defamation. Let’s say the movie has an evil drug dealer named Joe Thompson. If some guy named Joe Thompson in Wayzata, Minnesota decides the movie has defamed him and tries to sue, the studio can point to that disclaimer and say, "Look, we said this character wasn’t based on anyone."
Since you seem to like to watch the credits – God bless you – pay close attention, because that disclaimer isn’t always exactly the same. In the case of a movie like ALI, many of the characters obviously DID exist, and a lot of the events portrayed in the movie DID happen. So the disclaimer at the end might say something like, "This story is based on actual events. In certain cases incidents, characters and timelines have been changed for dramatic purposes. Certain characters may be composites, or entirely fictitious."
Among recent movies, CHICAGO, ADAPTATION and CATCH ME IF YOU CAN are based on true stories, but each has taken considerable dramatic license. You’ll see that reflected in the disclaimers at the end.
When you listen to DVD commentary tracks, you’ll often hear that a character was "inspired by" a real-life person or events. For instance, GO features a telepathic cat named Huxley, who is based on my friend George’s telepathic cat named Huxley. The threeway, the strippers and the burning hotel room all happened – at different times, to different people – but in stringing them together, I created a fictious work that is not really "based on actual events."
By the way, the screenwriter gets no say in what kind of disclaimer is put on the movie. That’s generally handled by the studio lawyers.