Inspired by a comment in this question, which reads as follows:
It says you must spend your action each turn, but it does not say that if you do not spend an action that the spell fails
And the rules for spells with long casting times are as follows (emphasis mine):
Certain spells (including spells cast as rituals) require more time to cast: minutes or even hours. When you cast a spell with a casting time longer than a single action or reaction, you must spend your action each turn casting the spell, and you must maintain your concentration while you do so (see "Concentration" below). If your concentration is broken, the spell fails, but you don't expend a spell slot. If you want to try casting the spell again, you must start over.
—Casting Time (Player's Handbook, pg. 202)
Is the comment accurate? If you don't spend your action on a turn casting the spell (but also don't spend your action otherwise), does the casting fail (even if you intend to continue spending your actions on the following turns)? Or does it only fail if concentration is broken?
Since the Player's Handbook quotation only specifies that the casting fails if the caster's concentration is broken, I'm wondering if the
you must spend your action each turn casting the spell
is another way that the casting can fail. To avoid this question being a duplicate of the one I linked, I want to know in a general case how this interacts. The "must" seems to imply that the caster can't just choose to "pause" their casting for a round, but what about being unable to use their action in some other way?
Best Answer
If you don't spend your action casting the spell, you have stopped casting the spell
There are only a couple of possibilities for what could happen if you start casting a spell with a long casting time and then don't use your action to continue casting it:
I can't think of any other reasonable interpretations of the rule besides these 3.
Option 1 is directly and unambiguously at odds with the rules, which say that "you must spend your action each turn casting the spell". So we can eliminate it right away.
Option 2 seems plausible, but there are a lot of problems you need to address if you use this interpretation. For example, how many rounds in a row can you "pause" the casting before the spell is lost? Could you pause casting a spell 1 round before you finish, and then wait indefinitely for the opportune time to complete it, thereby allowing you to set a trap with a spell that normally has a prohibitively long casting time? In short, Option 2 implies a whole new set of possible mechanics relating to "paused spellcasting" and its interactions with other mechanics, none of which are addressed at all in the rules. That makes it very unlikely that this is the intended reading.
So, having eliminated those two options, the only reasonable interpretation left is Option 3: the spell's casting is cancelled. This is certainly the most literal reading of the rule: if you spend your action each turn casting the spell, you cast the spell; if you don't spend your action each turn casting the spell, you don't cast the spell. The rules don't say that the spell fails if you don't use your action to cast the spell, because they don't need to. The spell doesn't fail: you simply don't cast it because you stopped casting it. The requirement to maintain concentration through the full casting time is an additional requirement to cast the spell and is unrelated to the requirement to spend your action each turn casting it.
(I agree that this logic gives an absurd result when combined with the rule that a surprised creature cannot take an action on the first turn of combat, but I would argue that the fault lies with the surprise rules, not the spellcasting rules.)