When Fizz is played, when do you choose targets for the spell played from trash? Is Fizz's effect a trigger which targets a spell in trash which resolves before the spell then goes on the stack?

To clarify how Fizz, Trickster works, here is the step-by-step breakdown of the timing and targeting:

1. Targeting Timing

According to RiftJudge FAQ #6132 and FAQ #9214, you target the spell in your trash when you put Fizz's triggered ability on the chain.

You must declare the spell you are choosing from your trash at the moment you add Fizz's "When you play me" (WYPM) ability to the chain.

2. Is Fizz's effect a trigger?

Yes. Fizz's ability ("When you play me...") is a triggered ability. It follows this sequence:

1. Play Fizz: You play Fizz from your hand. He enters the chain as a pending item.
2. Fizz Enters Play: Fizz finalizes and enters the battlefield.
3. WYPM Trigger: Because Fizz is now in play, his WYPM ability triggers and is placed on the chain. This is when you declare the target spell in your trash.
4. Chain Resolution: The chain resolves. Fizz's ability resolves, and you play the chosen spell from your trash (paying its Power cost, but ignoring its Energy cost).
5. Spell Finalization: The spell you just played now enters the chain as its own, new item. It will resolve once the chain resolves down to it.

Summary

Fizz's ability does target the spell in your trash, and that choice happens as the ability is added to the chain (not on resolution). The spell you choose is not placed on the chain immediately; rather, you play it as part of the resolution of Fizz’s ability, at which point it becomes its own chain item that will eventually resolve.

This is my interpretation based on the mechanics of triggered abilities and targeting as clarified in the retrieved RiftJudge FAQs. No official core rule contradicts this, and it aligns with the documented behavior for Fizz, Trickster.