Can frightened creatures stand up without "moving closer to the source of their fear"?

Question

An interesting problem came up in game the other day:

A Battle Master fighter (PHB p. 73-74) successfully used the Menacing Attack maneuver against a fire giant, and thanks to a magical effect on her blade, she knocked the giant prone.

She then used her bonus action to Dash and climb into the prone giant's chest. We're using the optional "Climb onto a Bigger Creature" rule from DMG p. 271:

As an alternative, a suitably large opponent can be treated as terrain for the purpose of jumping onto its back or clinging to a limb. After making any ability checks necessary to get into position and onto the larger creature, the smaller creature uses its action to make a Strength (Athletics) or Dexterity (Acrobatics) check contested by the target’s Dexterity (Acrobatics) check. If it wins the contest, the smaller creature successfully moves into the target creature’s space and clings to its body. While in the target’s space, the smaller creature moves with the target and has advantage on attack rolls against it.

The smaller creature can move around within the larger creature’s space, treating the space as difficult terrain. The larger creature’s ability to attack the smaller creature depends on the smaller creature’s location, and is left to your discretion. The larger creature can dislodge the smaller creature as an action—knocking it off, scraping it against a wall, or grabbing and throwing it—by making a Strength (Athletics) check contested by the smaller creature’s Strength (Athletics) or Dexterity (Acrobatics) check. The smaller creature chooses which ability to use.

For Menacing Attack, the rules state the following:

...On a failed save, it is frightened of you until the end of your next turn.

For Frightened, the rules state the following:

• A frightened creature has disadvantage on ability checks and attack rolls while the source of its fear is within line of sight.
• The creature can't willingly move closer to the source of its fear.

Here's the question: Would the action of standing up (with the source of fear literally on you) count as willingly moving closer to the source of fear?

And more generally, even without the 'standing on the chest' part, does standing up count as moving towards something?

Answer 1

The giant can stand up without worrying about the fear.

From the point of RAW you should treat being prone as a condition and spending movement as a cure:

1. The giant has the Proned condition.
2. Giant spends half of his movement (as a resource) to remove the Proned condition.
3. As the giant doesn't spend any of his movement to move, he is not moving.

If you are using a battle map with grids this is even easier to determine. If a target's threat area (5x5ft for medium, 10x10ft for large and so on) doesn't move -vertically or horizontally- your target is not moving.

After you determine that rule it's up to your DM to add some flavor to it. Either by altering the ruling a little, make it more realistic (you can't get up from the ground without moving after all) or finding an epic way to describe what the mechanic says.

Answer 2

This is not addressed in the rules because the rules do not allow your scenario.

The problem is that creatures cannot willingly end their turn in another creature's space (PHB 191):

Whether a creature is a friend or an enemy, you can’t willingly end your move in its space.

Therefore, strictly according to the rules, your fighter/rogue could not have climbed onto the giant's chest and stayed there, because they can't willingly end their move in the giant's space.

DM ruling? The giant can probably stand up

Given that the rules alone cannot fully address your scenario, you're in the realm of DM rulings.

Generally, assuming that nobody's flying, I don't think standing up counts as willingly moving closer to the source of fear--assuming you're using a grid, standing up doesn't change the square you're in, so there's no net movement.

Moreover, it would seem that the fighter/rogue would probably fall off the giant's chest when it stands up (maybe you can have them roll a save to keep holding on). Causing the fighter/rogue to fall off of its body would cause them to move further away from the giant, which is fine. Note that the frightened condition does not prevent a frightened creature from trying to push the source of its fear away.