Even worse is Mel-Gibson-Syndrome, when an actor feels "his repsonsibility" to make the movie better by writing his own scenes into the movie (ala Payback), or getting a director fired because he cut too many of your scenes and forcing the studio to allow you to edit your own cut of the film (ala Edward Norton).
If you are paid to act, then act. Leave the writing to the people who get paid to do it.