Complex sentence: Prouty served as a technical adviser to Oliver Stone's movie "JFK" and was the inspiration for the mysterious "X" who is based on him as well, played by Donald Sutherland, who assists Jim Garrison in the movie.
Split into simpler sentences: Prouty served as a technical adviser to Oliver Stone's movie "JFK". He was the inspiration for the mysterious "X" (played by Donald Sutherland), who assists Jim Garrison in the movie.