Você não declara suas ações de antemão; você apenas os pega.
Capítulo 9 do PHB / regras básicas descreve como as ações em combate funcionam.
A barra lateral "Combate passo a passo" no "A ordem do combate" A seção lista como o combate ocorre:
-
Determine surprise. The DM determines whether anyone involved in the combat encounter is surprised.
-
Establish positions. The DM decides where all the characters and monsters are located. Given the adventurers' marching order or their stated positions in the room or other location, the DM figures out where the adversaries are--how far away and in what direction.
-
Roll initiative. Everyone involved in the combat encounter rolls initiative, determining the order of combatants' turns.
-
Take turns. Each participant in the battle takes a turn in initiative order.
-
Begin the next round. When everyone involved in the combat has had a turn, the round ends. Repeat step 4 until the fighting stops.
Como você pode ver, não é declarado em nenhum lugar nesta macro que você declara suas ações antes de tomar a sua vez.
Olhando para o "Sua vez" Na seção, podemos ver o que você pode fazer no seu turno:
On your turn, you can move a distance up to your speed and take one action. You decide whether to move first or take your action first. Your speed--sometimes called your walking speed--is noted on your character sheet.
The most common actions you can take are described in the Actions in Combat section. Many class features and other abilities provide additional options for your action.
The Movement and Position section gives the rules for your move.
You can forgo moving, taking an action, or doing anything at all on your turn. If you can't decide what to do on your turn, consider taking the Dodge or Ready action, as described in "Actions in Combat."
A seção continua descrevendo ações de bônus:
Various class features, spells, and other abilities let you take an additional action on your turn called a bonus action. [...] You can take a bonus action only when a special ability, spell, or other feature of the game states that you can do something as a bonus action. You otherwise don't have a bonus action to take.
E outra atividade no seu turno:
You can communicate however you are able, through brief utterances and gestures, as you take your turn.
You can also interact with one object or feature of the environment for free, during either your move or your action. For example, you could open a door during your move as you stride toward a foe, or you could draw your weapon as part of the same action you use to attack.
If you want to interact with a second object, you need to use your action. Some magic items and other special objects always require an action to use, as stated in their descriptions.
E até reações:
Certain special abilities, spells, and situations allow you to take a special action called a reaction. A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else's. The opportunity attack, described later in this section, is the most common type of reaction.
... Mas, como você pode ver, em nenhum lugar de nenhuma dessas regras afirma que você deve declarar a ação que pretende tomar a qualquer momento; apenas diz que você as aceita (algumas a qualquer momento, outras quando uma determinada condição é atendida ou quando um recurso / feitiço permite que você as pegue).
Mais tarde no "Movimento e posição" seção, ele mostra como o movimento funciona:
On your turn, you can move a distance up to your speed. You can use as much or as little of your speed as you like on your turn, following the rules here.
[...]
You can break up your movement on your turn, using some of your speed before and after your action. For example, if you have a speed of 30 feet, you can move 10 feet, take your action, and then move 20 feet.
Você pode até se mover entre ataques:
If you take an action that includes more than one weapon attack, you can break up your movement even further by moving between those attacks. For example, a fighter who can make two attacks with the Extra Attack feature and who has a speed of 25 feet could move 10 feet, make an attack, move 15 feet, and then attack again.
Novamente, não há menção de declarar suas ações.
Como você pode ver, não há nada nas regras que exija que você declare suas ações de antemão - basta fazer as coisas que deseja fazer no seu turno, se tiver a ação de movimento, ação ou bônus (ou qualquer outro recurso necessário) ) disponível para isso.
O designer de regras Jeremy Crawford confirma oficialmente esse fato no Twitter:
D&D combat is sequential, with no action-declaration phase at the beginning. Your turn can also be interrupted by someone’s reaction. Such an interruption could, among other things, incapacitate you, meaning your intention to take a certain action was never fulfilled.
Ele acrescenta, em resposta a uma pergunta sobre a ordem de combate:
There is, indeed, tons of flexibility in how certain things can be ordered in combat. But if one thing is conditional on another, they must happen in order, for intent has no weight in the combat rules, since you could be interrupted at any moment and incapacitated.
Ele repete aqui:
D&D combat doesn't have an action-declaration phase. Things happen in order, and you can be interrupted at any moment by a reaction, trap, or the like. You can say, "I plan to take the Attack action," but that has no rules relevance until you're actually taking the action.
E mais uma vez, em resposta a uma pergunta sobre o Flurry of Blows do monge:
In D&D, the way you take an action in combat is to actually take the action. There is no action-declaration phase. Flurry of Blows happens after the Attack action, which means the action itself, not a declaration that you will take the action.