Does the chain start when playing a spell with no keyword on your turn outside of a showdown, and can opponents react to it with cards like Defy?

Ruling: Yes, the chain starts when you play a spell with no keyword on your turn, even outside of a showdown.

Nuances:

• Opponents can use cards like Defy to cancel the spell even if played before entering a showdown
• The statement that players can only react during showdown is incorrect