"She's not going to keep it secret, but she's not going to blab it in a way that would panic people, either," Harry answered with a shrug. "And we trust her enough to believe she's not going to say anything daft, or try to talk us out of what we're doing."
"We can tell her more," Harry agreed. Snape could get the over view, McGonagall the details. "She won't argue about it. Not really." McGonagall was always there, and willing, and brilliant - but she didn't interfere. Or she never really had. She'd taken her lead from Dumbledore, and Dumbledore had always left it to Harry to deal with on his own, most of the time. Not that Harry wouldn't love to have his help still.