I'm using simultaneous initiative (with an action declaration phase) and one-minute combat rounds for In the Realm of the Nibelungs, both a first for me and my group. Combat mostly runs via Swords & Wizardry Continual Light, a D&D 'retroclone', albeit with some changes (such as simultaneous initiative).
One question which has come up is "What, if anything, can combatants do with the rest of their round if their action is clearly resolved at the very beginning?"
During yesterday's session I, as a DM, declared that a wicker child would shoot its bow* at the cleric. The player declared his cleric would leap behind a tree (which would provide a cover bonus to Armor Class).
*It actually threw a spear, but for purposes of my question, I'll assume a missile attack with a bow. An archer gets only one attack roll, but I imagine him or her shooting multiple arrows in that time frame, with one 'meaningful' attack. If the need to track ammunition arose after the fight, I'd probably rule that 1d6 arrows were used per round. If it arose before due to limited arrows etc., I'd assume a single careful shot.
Anyway, I ruled that the missile attack would not automatically go first because the cleric was standing next to a tree. I had made this ruling in the prior session: 10' or less and you have a chance to move first via winning D&D's d6 group initiative roll; else, missile attacks are resolved before movement.
The attack was resolved - I don't remember if it hit or not, but the cleric lived - and the question came up, mostly post-game, whether combatants can do anything with the rest of their round in such situations. I'm inclined to stick with my no, as I think there should usually be only one action / endeavour per round. However, even going with that, the additional question comes up on how to narrate this. What's the cleric doing after leaping behind the tree? What's the archer doing? Or should they get some kind of second action after all?
(Using simultaneous initiave has been - and still is! - an exciting journey. I'm very pleased how all manner of positioning and initiative questions have fallen by the wayside - and almost imperceptibly so! - as common sense can be used to judge situations. Still, it has taken me a long time to wrap my head around it and my journey is clearly not at an end. In any case, today's question is mostly about one-minute rounds.)