Timing of held actions

[Bolo]

Suppose you're faced with an enemy whose strategy is to pop up from beneath the battlements, shoot an arrow at you, then duck back down out of sight again. He repeats this each round, so he's never visible when it's your initiative.

The solution is to hold your action until you see him, so that you're casting fireball on his initiative, not yours.

Now, when he pops up into sight, does his arrow go first, or does your fireball, or are they simultaneous? Or is this unspecified in the 5e rules and up to the DM to decide?

Or is this just the same as any other time when there's a tie between initiative rolls? For which the PHB says:

If a tie occurs, the DM decides the order among tied DM-controlled creatures, and the players decide the order among their tied characters. The DM can decide the order if the tie is between a monster and a player character. Optionally, the DM can have the tied characters and monsters each roll a d20 to determine the order, highest roll going first.

[Ginger]

I think the reaction is first. I think there is no "pop up and attack" action. A character moves, the reaction is triggered and occurs, then the character takes his action.

It can get more granular depending on the specificity of the held action. If I say, "I shoot anyone in this broad field who appears" that might imply a second to scan the area. If you say, "I have arrow nocked and will loose at whatever pops out around that corner," then I think you'd more clearly get the first shot.

[Bolo]

Got it. "Pop up and shoot" isn't an atom. The "pop up" is free, but it has to happen first, before the "and shoot". So the fireball goes off between "pop up" and "shoot". Thanks.

In 1e, where spells take time to cast, this would make no sense. The archer would appear, and then the caster would start casting, and surely the archer would release his arrow faster than the casting time of the spell. A key point I hadn't thought about is that in 5e, actions are instantaneous. I can run 30 feet in a turn, or I can run 30 feet and shoot an arrow, or I can run 30 feet and cast Fireball. There's no question of how long it take the caster to get off his spell. It just goes off immediately and instantaneously, as soon as it's triggered.

[Ginger]

Everything is just so much faster. A round is 6 seconds instead of 60, which creates so many changes in the feel of a combat.

1e doesn't even have a formal mechanism for holding actions. I think setting for charge is the closest.