Can a surprised creature fall prone voluntarily on their turn?

Question

Suppose someone might want their character to drop prone while they're surprised. The relevant text from Being Prone under "Movement and Position" states:

You can drop prone without using any of your speed.

(Notably this means that any effect which drops your speed to 0 does not prevent you from dropping prone.)

Surprise meanwhile stops us from moving or taking actions:

If you're surprised, you can't move or take an action on your first turn of the combat

I suspect this prevents a character from being able to drop prone, because preventing movement is different to simply reducing speed to zero. If my speed is 0, I can still drop prone, draw a weapon, or take other free object interactions as part of my move. If I can't move, I can't do any of these things.

Can a surprised creature drop prone voluntarily on their turn?

Answer 1

No, a surprised character can't drop prone

As you posted, Surprise says:

If you're surprised, you can't move or take an action...

This "can't move" terminology is actually used a lot in the rules. A few other examples:

Petrified:

The creature is incapacitated (see the condition), can't move or speak, and is unaware of its surroundings.

Stunned:

A stunned creature is incapacitated (see the condition), can't move, and can speak only falteringly.

Unconscious:

An unconscious creature is incapacitated, can't move or speak, and is unaware of its surroundings

The book also uses "speed reduced to 0" fairly often:

Exhaustion:

Speed reduced to 0

Grappled:

A grappled creature's speed becomes 0, and it can't benefit from any bonus to its speed.

Restrained:

A restrained creature's speed becomes 0, and it can't benefit from any bonus to its speed.

I think there is a clear distinction here. Petrified, stunned, and unconscious all describe someone who is not only incapable of moving, but incapable of even trying to move. Exhaustion, grappled, and restrained on the other hand don't prevent you from trying to move.

So, clearly the game thinks that there is a difference between being unable to move, and having 0 speed. It wouldn't make sense for someone who is Petrified, Stunned, or Unconscious to be able to choose to drop prone, while it would make plenty of sense for someone who is exhausted, grappled, or restrained to fall prone, or at least stop trying to stay upright.

Looking at it from another angle, you might argue that falling prone when you're surprised makes sense, as it's a very quick thing that you might do without knowing exactly what danger is at hand.

However, that argument falls apart because being Surprised also prevents reactions. Reactions such as the Shield spell, which happens so quickly that you can decide to do it between being hit by an attack and actually taking damage from said attack.

Ultimately however you look at it, no movement means no movement.

Answer 2

If it requires any movement at all, then a surprised character can't voluntarily drop prone. Ask your DM if they rule that the movement or positioning of falling prone has to require appreciable effort or if it has to require any movement at all.

That a surprised character can't voluntarily drop prone, is covered in the following four paragraphs. I then immediately go into explanations on why each step is relevant to the terminology. After that, I propose a different reading of the rules that goes into the convoluted argument that you can drop your weapon without making a free object interaction which has resonance in this thread and likewise that you could argue that falling prone is no movement at all because there is no appreciable effort required to fall prone. At last, I raise the argument that even if you consider falling prone to take no movement at all you may not have the awareness to fall prone if you are surprised (this introduction is inspired by linksassin). Now to the argument:

Speed only determines how far you can move.

Speed, PHB 181:

Every character and monster has a speed, which is the distance in feet that the character or monster can walk in 1 round. This number assumes short bursts of energetic movement in the midst of a life threatening situation. The following rules determine how far a character or monster can move in a minute, an hour, or a day.

But the condition Surprise is explicit in that you can't move, at all, logically you can not do any of the Movement and Positioning under which Being Prone is listed (PHB 190).

There is a distinction in terminology that is made regarding Conditions, PHB 290-292.

Petrification lists the distinct states, Petrified, PHB 291:

The creature is incapacitated (see the condition), can’t move or speak, and is unaware of its surroundings.

Distinctions are:

1. Incapacitated.
2. Can’t move.
3. Can't speak.
4. Is Unaware (of surroundings).

Being grappled further indicates that a bonus to a creatures speed would resolve the issue of having 0 speed, it could, therefore, move.

Grappled, PHB 290:

A grappled creature’s speed becomes 0, and it can’t benefit from any bonus to its speed.

Having 0 speed and not being able to move is not the same. The different states of being petrified showcase the difference in the agency that is left to you.

Is there a case for you being able to drop prone without being able to move?

Yes, there is!

There are two philosophies, according to this one you can drop your weapon and not count that weapon as an object and therefore make no object interaction because

letting go of something requires no appreciable effort. But picking it up does.

As argued in this thread, the quote refers to this Tweet by Jeremy Crawford.

(The other is that weapons are objects)

So if letting go of an object costs you no object interaction because there is no appreciable effort required, then you could argue that there also is no appreciable effort required from moving from standing into dropping down - merely relax your muscles.

This perspective follows the Condition of being Unconscious, PHB 292:

The creature drops whatever it’s holding and falls prone.

Here dropping whatever (including weapons - whether they are objects or not)) is treated in the same manner as falling prone, indicating that there is some effort required in standing up.

Even if you do not count falling prone as a movement being Surprised also prevents reactions. Reactions such as the Shield spell, which happen so quickly that you can decide to cast Shield between being hit by an attack and taking damage from that attack. (Thank you AgentPaper for pointing this out explicitly)

What Surprise does not do, is: make you unaware (of surroundings) - compare to Petrified above. Therefore strictly reading: if you buy into the "no appreciable effort required" argument, then you would be able to voluntarily fall prone under the condition Surprised.