To elaborate: The Captain can steal “Doctor, Engineer, Scientist, or Expendable Crewman.” The Expendable Crewman’s card states that any time a player takes away one of your Keepers, “they MUST take this one instead.” So, does the Captain have a choice or MUST the Captain take the Expendable Crewman first?
A: Well, The Captain can steal any of those cards, but they might not all be owned by the same player. You are only forced to take the Expendable Crewman if the person you are stealing from has that card.
So if Player A has the Captain,
and Player B has the Doctor,
and Player C has the Engineer and the Expendable Crewman
…then player A can take the Doctor from player B, but they cannot take the Engineer from C – they must take the Expendable Crewman instead.
Of course, if Player A takes the Expendable Crewman on their first try, then on their NEXT turn, they can take the Engineer if they want (assuming they still have The Captain).
Note that in Star Trek Fluxx, Ensign Smith functions as the Expendable Crewman, while in TNG Fluxx, Tasha Yar has that special ability.