Trying to answer a question with Hermione next to him, bobbing up and down on the balls of her feet with her hand in the air, was very off putting, but Harry had a go.
“Er—because there are so many of us, it won’t know what shape it should be?”
“Precisely,” said Professor Lupin, and Hermione put her hand down, looking a little disappointed. “It’s always best to have company when you’re dealing with a Boggart. He becomes confused. Which should he become, a headless corpse or a flesh eating slug? I once saw a Boggart make that very mistake—tried to frighten two people at once and turned himself into half a slug. Not remotely frightening.
“The charm that repels a Boggart is simple, yet it requires force of mind. You see, the thing that really finishes a Boggart is laughter. What you need to do is force it to assume a shape that you find amusing.
“We will practice the charm without wands first. After me, please… Riddikulus!”
“Riddikulus!” said the class together.
“Good,” said Professor Lupin. “Very good. But that was the easy part, I’m afraid. You see, the word alone is not enough. And this is where you come in, Neville.”
The wardrobe shook again, though not as much as Neville, who walked forward as though he were heading for the gallows.
“Right, Neville,” said Professor Lupin. “First things first: what would you say is the thing that frightens you most in the world?”
Neville’s lips moved, but no noise came out.
“—didn’t catch that, Neville, sorry,” said Professor Lupin cheerfully.
Neville looked around rather wildly, as though begging someone to help him, then said, in barely more than a whisper, “Professor Snape.”