Shutter Island is a period piece with nods to different films in the film noir and horror genres, paying particular homage to Alfred Hitchcock's works.[15] Scorsese stated in an interview that the main reference to Teddy Daniels was Dana Andrews's character in Laura, and that he was also influenced by several very low-budget 1940s zombie movies made by Val Lewton.[16] The main frame of the plot resembles that of William Peter Blatty's The Ninth Configuration,[17][18][19] as well as The Cabinet of Dr. Caligari.[19][20][21] La Croix noted that Shutter Island was a "complex and puzzling" work that borrowed from genres as diverse as detective, fantasy, and the psychological thriller.[22]
There have been differing opinions over the ending of the film, in which Laeddis asks Dr. Sheehan, "which would be worse – to live as a monster, or to die as a good man?", a line that does not appear in the book. Professor James Gilligan of New York University was Scorsese's psychiatric adviser, and he said that Laeddis's last words mean: "I feel too guilty to go on living. I'm not going to actually commit suicide, but I'm going to vicariously commit suicide by handing myself over to these people who're going to lobotomize me."[23] Dennis Lehane, however, said, "Personally, I think he has a momentary flash.... It's just one moment of sanity mixed in the midst of all the other delusions."[23]
