... Never mind. So why would one of the weirdest actors in Hollywood be cast as one of the sweetest detectives in literary history? Maybe so that this character finally makes sense.
Eccentric fictional detectives -- your Miss Marples, your Mentalists, your Sherlock Holmeses -- prefer to employ an old-fashioned criminological technique called "guessing." Using supernatural deduction skills, they somehow manage to get murderers to confess simply by repeating their crimes back to them in exact detail, like it's a game of Detective Hero wherein tapping out a perfect score automatically unlocks the prison bonus round. Though typically, the criminals then either commit suicide on the spot or get killed by an angry lover. It saves everyone a lot of paperwork. And mistrials.
The problem with this trope is that these clever fictional detectives are often also very lovely people. They're sophisticated, they're genteel, they'll never racially profile. Poirot is the best example of this, an old man with a silly mustache and a bowler hat milling around country estates charming murderous aristocrats with his accent.