In a showdown, can reactions only be played as a response to actions, or can both actions and reactions be played by any player during this time?

Ruling: Reactions can be played at various times - either to open a chain when you have Focus, or in response to Actions, Reactions, or triggered abilities when you have priority.

Nuances:

• Reactions are not limited to only responding to actions
• The timing depends on whether you have Focus (to open a chain) or priority (to respond to something on the chain)