How to manage using Ready for both Movement and Attack?

Question

Here's a specific situation: a rogue is waiting around a corner for an enemy to move in sight. The enemy will pass about 10 feet away from the point the rogue is in currently. The rogue wants to Ready so that when the enemy is 10 feet away he can emerge from hiding, move 5 feet (he has at least 5 feet of movement left) and stab the enemy with his shortsword in order to make use of his Sneak Attack.

How should this situation be handled, since Ready says that you can Ready an Action OR a Movement, even though it seems reasonable that he should be able to do it?

Answer 1

A different way to solve your problem: throw a dagger at your target

Your sneak attack damage will only trigger on one attack during your turn (p. 27, Basic Rules), and you have to choose between move or attack for a Ready action (see Action Economy, below).

Ready a thrown dagger attack to fulfill your intent.

Why a dagger rather than my short sword?

The dagger has the thrown property, the light property and finesse property, all of which fit the criteria for a sneak attack in this situation. You are within range, so you do not get disadvantage on the thrown attack. Were the target 25 feet away, this would not be a good idea, as the disadvantage would remove one of the requirements for your sneak attack to trigger unless the target was also adjacent to an ally when you throw the dagger. (p. 27, basic Rules)

Dagger / 1d4 piercing / Finesse, light, thrown (range 20/60) (p. 46 Basic Rules)

The difference in damage between 1d4 + xd6 and 1d6 + xd6 is on average one damage point. (1d4 averages 2.5 and 1d6 averages 3.5). The larger part of your damage comes from your sneak attack bonus as your levels increase.

• Example damage, sixth level Rogue:
  1d4 + 3d6 averages 2.5 + 10.5 = 13
  1d6 + 3d6 averages 3.5 + 10.5 = 14
  If the DM holds that your attack is a surprise, and you have the Assassin rogue archetype, this makes for a substantial damage bonus: you double it all. (See Critical Hits, Basic Rules, p. 75).

Per the tweet from Jeremy Crawford

Jeremy Crawford : When you use Ready, you prepare an action or a move, not both

This matches the rules in the book:

... choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. (PHB, p. 193)

The Action Economy at work

Since your reaction is happening during another creature's turn, you don't get all of your options for doing something that you get on your normal turn: action, bonus action, movement, free interaction. (Basic Rules, p. 69-70). All you are able to apply is your reaction so you need to choose to either attack or move.

For this situation, your best bet is to attack with a thrown dagger since your intent is to make an attack as a reaction to the target moving.

Answer 2

Ready is for Action OR Movement - not both

It may seem reasonable to you, but the rules are clear (PHB, 193) - emphasis mine.

First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.

This was clarified through Jeremy Crawford via Twitter as well (and amazingly, Mike Mearls is in agreement for two designers confirmation)

Jeremy Crawford : When you use Ready, you prepare an action or a move, not both

At Your Table

You can always choose to create a House Rule allowing both if you like (just like with everything else.)

Answer 3

As per RAW (Rules As Written), you cannot move AND attack when using the ready action, as the PH, p.193, says the following:

Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.

The reason for this is that, since you are using your reaction to, well, "react" to the trigger, you can move again using your reaction, not your movement. So, technically, you could move your full speed during your turn, and then move your full speed again as a reaction.
Mind you, there might be some clause hidden somewhere in the PH, on Sage Advice, or at some other place that describes this more detailed and potentially says something different - if so, I'm not aware of it and would be happy about any hint about it.

Anyways, while you - as per RAW - can't take both an action and move as part of a readied reaction, your DM (or you, if that's you) could always houserule in this regard, such as saying that:

[potential house rule] When you take the ready action, you can move half your speed (or your remaining speed, whichever is lower!), and take a regular action. Alternatively, you can move up to your full speed (regardless of how far you moved during your regular turn), but you cannot take an action.

If you do use this (or a similiar) house rule, you should notify your players that you might change it later, depending on how powerful it turns out to be.

Also note that, since one takes his reaction for this, other things that one can usually do using one's reaction become unavailable until after one's next turn. Examples include making an opportunity attack or casting the shield spell.