What separates a willing follower from a useful idiot?
Short answerA willing follower knowingly accepts the task. The useful-idiot interpretation applies when someone's conception of their role makes them obey predictably even while disagreeing.
NotesA person's intelligence or professional competence does not settle this distinction. Someone who knowingly accepts a task can simply be a follower. A person who cannot imagine departing from an assigned role is more predictable. Political leaders can rely on that person to carry out their wishes even when he objects to their views.