Exorcist, The Easter Egg - Author Cameo!

Near the beginning of the film, a movie is being shot at the Washington University starring Sharon (the Mother). At the very start of that scene, the director Burke Dennings steps out of his trailer onto the location where a man with a mustache holding the script asks if "this scene is really necessary?” That person is the actual author of The Exorcist and original screenplay writer for the film, William Peter Blatty.
It is an interesting egg because Blatty is asking the fictional director about what scenes to include, when in real life the director of The Exorcist (William Freidkin) and Blatty actually did dispute for months over what scenes should and should not be cut from the final version of the film in 1973. These 'disputes' then fueled the release of 'The Version You've Never Seen' as part of the films 25th Anniversary in 1999, where some scenes were put back in.