Strange still tries to remain calm and smug, trying to seem like he has the slightest idea about what he's doing when in reality he does not have a goddamn clue in the slightest what he's doing. So, he lets Childermass get all up in his space and just glares at the changeling

"I know what I'm doing," Strange hisses, with a scowl. "And as for the changeling point, that's not going to happen. Has it even occurred to you that the Summer Court might have use for a human ally?"

He won't be a changeling, he'll just be brainwashed, totally different things, gosh Childermass.