Are you setup so that the actual 'Player' is an empty/invisible object that sends commands to the characters or are you setup so that the controls and commands are handled locally on the characters?
Either way is very similar but you just need to add a state that says that this character is managed by AI or that it is just idled, then when you switch by clicking or whatever it just changes states to be in a state that has all of the controls and inputs being pushed onto the character. Or, that the commands from the 'player' are sent to that particular character/gameobject instead, so it would be a trick of defining the target object that the commands are sent to.
Its a matter of defining a "not controlled by the player" state and triggering it appropriately or changing the target object for commands.
