The Feign Death spell begins with this line:
You touch a willing creature and put it into a cataleptic state that is indistinguishable from death.
I read this as flavor, since ‘cataleptic state’ isn’t defined by the game. It is still a very evocative description of the spell effects. Here’s a definition of a catalepsy pulled from Merriam-Webster:
Medical Definition of catalepsy. : a trancelike state of consciousness (as that occurring in catatonic schizophrenia) that is marked by a loss of voluntary motion and a fixed posture in which the limbs remain in whatever position they are place
Feign Death goes on to explicitly describe the effects of the spell in DND terms.
The target is blinded and incapacitated, and its speed drops to 0.
Emphasis is mine
Importantly, none of these conditions or effects prevents a creature from talking, moving their limbs, thinking, hearing, smelling, making saving throws as normal, or making ability checks as normal.
As Jeremy Crawford has said, being incapacitated doesn’t immobilize you. Typically, ‘incapacitated’ is wrapped up inside more specific conditions or effects that describe movement. For example, the paralyzed condition incapacitates a creature and also prevent’s movement/speech:
A paralyzed creature is incapacitated (see the condition) and can’t move or speak.
It seems to me that the RAW version of Feign Death and it’s “cataleptic state” would allow a creature to spend an hour flailing it’s arms around, singing 99 bottles of beer on the wall, and making DEX saves against the odd fireball. Maybe this is RAI, too, since the spell only works on a willing creature, who would presumably lay still and play dead. Unless they’re in it to delay a disease, or rolling around to shield themselves from a fireball.
Is this absurd? Am I understanding the spell correctly?