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?