When Teemo's ability resolves (flip 5 cards and deal damage), does it all resolve at once, or can players react between the flipping and the damage?

Ruling: Teemo's ability resolves all at once. You cannot interrupt abilities while they are resolving.

Sequence:

• Teemo enters play or becomes a defender
• His ability goes on the chain
• Players have an opportunity to play Reactions before it resolves
• Once it begins resolving, the entire ability (flip 5 and deal damage) completes without interruption

Nuances:

• You can respond to the ability before it resolves, but not during its resolution
• Under current rules, defense abilities trigger when a unit becomes a defender for the first time in combat, not just at the start of combat