When an opponent plays a spell (action or non-action), does focus/priority pass to me so I can respond with a reaction?

Ruling: When your opponent plays a spell and passes priority, you can cast a reaction spell which will resolve before their spell resolves. Focus is only used during showdowns to start chains; priority is what passes around during an active chain.

Sequence:

• Opponent plays a spell (like Charm) and passes priority
• You receive priority and can play a reaction spell
• Your reaction resolves first
• Then the original spell resolves

Nuances:

• Focus and priority are different mechanics: focus is only for showdowns and is the ability to start a chain, while priority passes around during an existing chain
• You don't get focus to play actions outside showdowns, but you do get priority to play reactions when spells are on the chain