Can a player respond with an action spell when their opponent plays an action spell during a showdown, and can action spells be played when an opponent moves to an unclaimed battlefield?

Ruling: Players always have an opportunity to play an action spell in a showdown they're part of, allowing them to respond to opponent action spells.

Nuances:

• Both action and reaction spells may be played during showdowns on either player's turn
• The specific timing and interaction details are covered in referenced threads about showdown mechanics