Murder by the Book (1987)

Mild-mannered mystery writer D. H. Mercer has become so immersed in his material that his creation, hard-boiled private eye Biff Deegan, constantly appears to him as a hallucination. Intent on getting rid of Biff, and replacing him with a more civilized detective, Mercer soon finds himself in a genuine mystery involving art fraud, murder, and a beautiful lady in peril.