James doesn't mean to laugh, but Lily's being kind of ridiculous, and mostly ridiculously adorable — because here she is, about to be brilliant in her first wizarding interview, and she's nervous.

It's endearing.

And he doesn't laugh to be mocking.

He takes a breath, then places his hands on Lily's arms, to steady her.

"Lily," he starts, "you'll be brilliant. You look fine. No one expects you to dress like the rest of them do, besides which no one but our parents' generation really goes around wearing robes all the time anyway."

Case in point: himself. Not a single robe-like thing on him.

"You got this interview because you are brilliant. Not because of what you wear."