They might be giants (1971)

After the death of his wife, wealthy retiree Justin Playfair creates a fantasy world for himself in which he is the legendary detective Sherlock Holmes, even dressing like the character. Out of concern for Justin's money more than his health, his brother Blevins puts him under the care of psychiatrist Dr. Mildred Watson. As Dr. Watson grows fond of Justin, she begins to play along with his theories, eventually becoming an assistant in his investigations.