Can reactions be played during a showdown, and what is the difference between when you can play actions versus reactions?

Ruling: Reactions can be played anytime you have priority during a showdown, whether the chain is empty or not. Actions can only be played during a showdown when the chain is empty.

Sequence:

• When you have priority, you can play as many reactions as you want
• When you pass priority and nobody reacts, you don't get priority again until the first item on the chain resolves
• Focus passes automatically once a chain is fully resolved during a showdown
• The player with focus can play an action to start a chain, then immediately follow up with reactions without passing priority

Nuances:

• You cannot play action spells to an ongoing chain, only to open a chain
• You cannot follow up an action with another action even if priority passes back to you during the same chain
• Gaining focus also gives you priority
• Passing priority during a chain doesn't pass focus; focus only passes once a chain where you had focus is emptied or when you decide to not start a chain