If by this point you don't believe Celia is the same TMA universe Celia her meeting with Lady Mowbray cemented she is, because that was the most "seasoned Magnus Achives character meets an avatar" interaction we've had in a while:
- ask her if you can do something for her just to make sure she scrams
- don't give your name when trying to be tricked into saying it
- ignore her creepy sniffing and vauge remarks
- don't interact needlessly
- don't play into her narrative
- don't get caught on the stupid pun at the end
Godspeed Celia, you beautiful creature, if anybody is equipped to live till the end it's you.