For spells like Stupefy, Falling Star, FoxFire, and Void Seeker, do you choose targets on cast or on resolution?

Ruling: You choose targets when you play the spell and it goes onto the chain, not on resolution.

Nuances:

• Spells with "do this" language (reflexive triggers) are an exception - those delay when that part of the ability goes onto the chain and create new chain items that make choices when those triggers go onto the chain
• For anything not a reflexive trigger, pick targets when you play it
• This means opponents can react after targets are chosen (e.g., if you play Singularity and choose targets, then your opponent can react with Retreat)