You add in the Aces, with the ace of hearts being a detective, and the ace of diamonds being a doctor. After the mobsters finish picking someone they all sleep. Then the detective gets to point to someone and the moderator shows them a card, either a face card for a mobster or a normal card for a citizen.
Then it is the turn for the doctor. The detective goes to sleep, and the doctor gets to point to someone to save. If the doctor points to the person that the mob tried to kill, then that person is saved.
Neither the doctor or the detective can reveal that they are the doctor or detective, (for this you need to bring in a rule that nobody is allowed to ask).
This adds some extra intrigue to the game.