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 prevents 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 its “cataleptic state” would allow a creature to spend an hour flailing its 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?
EDIT: For those answers hinging on “indistinguishable from death”
Here is how that sentence reads, along with the immediately following one:
You touch a willing creature and put it into a cataleptic state that is indistinguishable from death.
For the spell's duration, or until you use an action to touch the target and dismiss the spell, the target appears dead to all outward inspection and to spells used to determine the target's status.
Clearly the spell states that the effect is “indistinguishable from death” to outsiders. I don’t believe it pertains to the target of the spell. After all, they are only blinded and unable to take actions (incapacitated). They aren’t deaf, unconscious, or paralyzed, or even stunned. All defined conditions. They can presumably distinguish their state from death, and I see no reason that they couldn’t move their limbs RAW.
What about the “cataleptic state.” Here’s where I want to defer to a ruling by Jeremy Crawford about the grease spell. A player asks if the grease from this spell was flammable, and Jeremy rules that if it were flammable, the spell would say it were flammable. I think we know that grease is flammable. Applying his ruling to this case, I’d say if the cataleptic state prevented movement and speech, the spell would say it prevents movement and speech. Or choose a condition that does. It doesn’t matter that “cataleptic state” has this property in our world.
Best Answer
The spell says, among other things:
There are a few relevant effects here.
1 - The creature is put into a cataleptic state.
A cataleptic state is, according to Merrian-Webster, "a trancelike state marked by loss of voluntary motion in which the limbs remain in whatever position they are placed".
This isn't optional or fluff - this is an actuall effect of the spell. Once you're affected by Feign Death, you lose voluntary motion. You're a ragdoll, not much else.
2 - Said state is "indistinguishable from death".
The state of loss of movement cited above is a bit special - not only the affected creature loses all voluntary movement but also becomes so still it is indistinguishable from death. Couples with the next item...
3 - The target appears dead to all outward inspection and to spells used to determine the target’s status
This makes so that inspection to check the actual state of the creature fails. One won't be able to detect its breathing, its vital signals, read its mind, and so on. Any attempt to determine if a creature is alive should give the answer that it is actually dead.
4 - The target is blinded, incapacitated, and its speed drops to 0.
Those are straightforward. The target can't take actions or reactions, can't see, and isn't able to move.
None of these effects are optional. All of them apply at the same time.
A creature under effect of Feign Death can't move from its square (since speed 0), can't speak, move, or move their limbs (since catalepetic state removing voluntary control), can't be detected as alive via spells or outward inspection (since the effect says so), is blind and can't take actions or reactions.
That said, the creature can still roll all of their checks (as nothing in those effects say they can't). This is very weird from a rules-as-intended standpoint, but on RAW that's what happens.
You certainly won't be able to speak or move, however. That is pretty clear from the first clause (cataleptic state), and as it is a described effect of the spell, it is non-negotiable.