Harry: Professor, can I ask you something?
Professor Lupin: You want to know why I stopped you facing that boggart, yes? I would have thought that would be obvious - I assumed it would take the shape of Lord Voldemort.
Harry: I did think of Voldemort - at first. But then I remembered that night on the train... and the dementor...
Professor Lupin: I'm very impressed. That suggests that what you fear most of all... is fear itself. This is very wise.