When attacking a Deflect unit with a 'when I defend' ability (like Teemo), does the defending player pay Deflect when the ability is put on the stack, or can they cast a reaction spell (paying Deflect for that) and then not pay for the defend ability?

Ruling: The defending player must pay Deflect when the defend ability is put on the stack, before any reactions can be played. All targets must be chosen and Deflect costs paid for abilities to finalize before reactions are possible.

Sequence:

• The defend ability attempts to add itself to the stack
• Targets are chosen and Deflect must be paid (or declined) for the ability to finalize
• Only after abilities finalize can players play reaction spells