Many years ago I saw a great movie about a police agent that is recruited because he is similarly looking to a killer that is in the prison. Criminal in the prison dies and police (don’t know if it was FBI/CIA/etc.) makes a cop learn his way of life to impersonate him to get information about some big deal. I think it may have been based on a historical actual killer (and I think he was redhead/maybe irish).
There was a scene when the agent comes to killer home and he needs to behave exactly like him so his girlfriend doesn’t know it’s not him.