Then when someone casts Fireball at me, I can use my reaction to cast the 4th level Resist Energy spell.
Does anyone remember if there are rules for exactly what order effects happen in when reactions are being used? I though there was. In fact, I think that for reactions, unless they specify otherwise, the effect of the reaction happens after the effects of the action that triggered it.
If I am remembering right, then doing this setup with Trickster's Ace and Resist Energy would still work, but the effect of the Fire resistance would only happen after the damage from the Fireball. So it would be useful for the next Fireball, but not the triggering one.
Usually before the triggering effect happens unless stated otherwise, e.g. AoO versus Movement in Enconters.
"leaving the square" => before you leave the square
"standing up" => after you finished standing up
Else reactions like AoO or Stand Still could never disrupt any actions.
OK. I may have a bit of edition confusion. I was probably thinking of the Starfinder Reaction rules. Which would still allow this scenario to work since Resist Energy would be considered a defensive reaction.
If I remember correctly there isn't a defined order of operations, but rather it's dependent upon the trigger. That being said, most of the examples I can find tend to happen either before or in the middle of an action rather than after it. I'm guessing there's at least some that trigger after the action completes, though... idk, I haven't really looked.
For Trickster's Ace in particular the trigger is decided by the player, which can cause weird things to occur if they try to use out of game knowledge, which is probably why they added the note that the GM might rule that the trigger's too complicated to work, so in this case I think it'd be up to the GM to decide if Resist Energy can be used to block the fireball or not. Personally, I would allow it if the trigger was something the character would be able to tell in an instant and react to, which for a fireball isn't unreasonable IMO.