Q: At what step in the attack sequence does Ultron Stage 2’s Forced Interrupt trigger?

A: Ultron Stage 2’s ability triggers immediately after he initiates his attack. That happens before defenders are declared. So the original target always gets the drone. (Developer Ruling, Hall of Heroes)

Q: Does Ultron Stage 2's attack get modified if, for instance, a boost results in a drone entering play?

A: Yes, if the number of drone minions engaged with you changes before Ultron’s attack resolves, then you will recalculate Ultron’s attack as a result. -(Developer Ruling, Hall of Heroes)

Q: I’m being attacked by Ultron Stage 2 and Nova is in play. Which interrupt triggers first?

A: The Forced Interrupt triggers before you can trigger a player Interrupt. -(Developer Ruling, Hall of Heroes)