When does targeting happen for spells and abilities, and can players react before targets are selected to reveal hidden units?

Ruling: Playing a spell or ability requires making all necessary choices including targets immediately. Players cannot play cards or abilities while another player is in the middle of playing a card or ability, so you cannot react before targets are selected.

Sequence:

• When you play a spell/ability, you must immediately select all targets as part of playing it
• The spell/ability then goes on the stack/chain with targets already locked in
• Other players can only react after the spell/ability is fully played (with targets already chosen)

Nuances:

• For Singularity: Cannot target a hidden unit even if revealed in response, since targets are chosen when the spell is played
• For Falling Star: If the opponent reveals a hidden unit while Falling Star is on the chain (before it resolves), you can target the newly revealed unit when Falling Star resolves and creates its damage abilities. However, if they reveal after the damage abilities are already on the chain, those abilities' targets are fixed and cannot change
• You cannot react to damage itself, but you can react to things that cause damage (with combat damage having the showdown phase for reactions)
• If a unit with deathknell dies to combat damage, you can react to the deathknell trigger before losing battlefield control