A Doctor - They can protect a person they want to. For example, let's say Morry is the doctor and she says (via a private message to me of course) "I choose to protect Racordany". Now let's pretend Morgoth & company wants to kill Racordany, however they won't be able to do that as he is immune, for that certain night anyway.
The Doctor also cant protect the same person in row, so for the next night she will have to protect someone else or no one. (Just to make it clear)