So you would make the assassin pre-declare their action before initiative is rolled?
Im not MAKING him do anything!
He is hidden from an orc walking down the corridoor, and he decides to attack the Orc with his bow and declares he is attacking,
triggering combat and inititive.
He isnt 'pre-declaring' anything. He tells me he shoots the Orc. Its his agency. That starts combat.
There is a chance that the Orc might be able to react fast enough to that attack to not be surprised.
What if the action the assassin had pre-declared was no longer suitable (e.g. someone else kills the target)?
Then you say: 'just before your arrow is loosed, you notice your target has already been felled; what do you do'?
Do you do this to all PCs or just assassins? Do you do this with NPCs as well?
Literally with everyone for every combat.
Like if your Wizard was chatting with some NPC's armed with crossbows, and they suddenly take aim and fire them at him, they dont get a free round of combat on the hapless PC. Initiative is rolled.
If the PC Wizard goes first, he gets a chance to act before they get those bolts off. He sees them take aim at him or catches the glint of evil in their eyes and manages to get a spell off before they pull the trigger.
Same deal if the Wizard decides to fireball them (triggering combat). The NPC's get a chance (assuming they rolled well enough on the Dex check) to shoot him before he finishes casting the spell and frying them.
This hasn't been true since 3rd edition did away with the "you must declare your action and then roll initiative" way of handling combat. You can still narrate it the old way if you want to, but this is fundamentally not how the game actually works anymore (unless your playing 2nd ed or earlier, playing an OSR clone or using optional rules/house rules).
The ONLY way combat can be initiatied is when someone declares a hostile action. Its not a case of asking everyone to pre-declare actions at the start of the round.
There was no round until the action was declared which started the cyclical turn based combat resolution sequence there. Its the very declaration of a hostile action that switches the game from narrative time to the combat round sequence. So he doesnt pre declare his first combat action (he doesnt have a combat action yet); his declaration simply initiates the start of combat.
You're running a game where the first guy to scream I ATTACK!!!! gets a free round of combat, or an infinite initiative score. I dont see that rule in the combat section anywhere. All declaring an attack does is end narrative time and begin the combat sequence as described in the PHB. Initiative is rolled and anyone that scores higher than the dude that started the ball rolling, gets an opportunity to act or react before that guy.
Unless they're surprised of course, in which case they dont get to act on the first round.]
Narrate it accordingly. Those are the rules.