Harry Potter: Professor, can I ask you something?
Remus Lupin: You want to know why I stopped you facing that boggart, yes? I thought it’d be obvious. I assumed it would take the shape of Lord Voldemort.
Harry Potter: I did think of Voldemort at first. But then I remembered that night on the train… and the dementor.
Remus Lupin: I’m very impressed. That suggests what you fear the most is fear itself. This is very wise.
From Harry Potter and the Prisoner of Azkaban (2004)
That doesn’t really make any sense though, as Lupin stepped in front of Harry to take the boggart AFTER it turned into a dementor, not before.