If I buff my Fiora to give her Deflect after my opponent has already cast a spell targeting her, does my opponent have to pay the Deflect cost?

Ruling: No. When a spell is finalized (targets chosen and costs paid), those choices are locked in and cannot be changed. Giving Fiora Deflect after a spell targeting her has already been played does not retroactively add a Deflect cost to that spell.

Sequence:

• Opponent plays spell targeting Fiora (without Deflect)
• Opponent finalizes the spell by choosing targets and paying costs (no Deflect cost at this time)
• Spell is now on the chain with locked targets and costs
• You respond by buffing Fiora to give her Deflect
• The original spell resolves without requiring Deflect payment
• Any NEW spells cast after Fiora gains Deflect would require the Deflect cost to be paid

Nuances:

• If you buff Fiora to 5 might (giving Deflect) and opponent casts two Stupefys simultaneously before either resolves, they pay Deflect twice
• If the first Stupefy resolves (removing Deflect), the second Stupefy would not require Deflect payment
• Reflexive trigger spells like Falling Star work differently - they create targeting effects on resolution, so buffing Fiora before the Star resolves (but after it's played) can force Deflect payment on the triggers