Imagine this scenario:
A stone giant is on cliff 40' up from the player characters. He is standing behind wall/embankment tall enough to conceal half his body.
While he is standing, he has cover against the players, but the players do not have cover relative to him.
I would like the Stone Giant to be able to bend down to grant him improved cover between turns. Not quite taking a knee, but just bending down. My rules questions is this?
Would bending down a little be a free action, a move action or a swift action? Can he use his 5' step?
I am the GM, so I can rule it either way, but I want to make sure this holds up against the rules. If questioned, I could rule that he uses his move equivalent action to stand back up, and a standard action to throw the rock. Then either a free action or a 5' to bend back down to give improved cover (I want him to still watch the battlefield).
Does this sound like it holds up? I want the players to be able to use this tactic as well with ranged weapons if they are in a similar situation. I plan on teaching them this tactic early on so when they see the stone giant scaling a wall high up for protection they know he will have a strong defensive advantage once in place and give gravity to trying to stop smart positioning.
I understand it this way:
You have 10 "free" points of AC (because your AC is 10 + all the other stuff). Those 10 points of "free" AC mean that you're doing all that you can to make yourself hard to hit. Sure, highly dexterous people do it better, but that's a separate part of your AC.
Also, a combat action fills six whole seconds. That's a long time. During those six seconds, you're moving, twisting, bending, ducking, doing whatever you can to A) hit the other guy to the best of your ability and B) avoid being hit.
That said, if a combat situation grants you more AC, you still do the very best you can to use that AC.
To directly answer your question, the bonus to AC the giant gets from his cover already assumes that he is bending into it for maximum effect, doing the best he can to avoid being hit.
In other words, if two giants are on that ledge and one of them is bending/squatting/ducking down to get Improved Cover, then what's wrong with the other giant? Is he a half-wit? Does the moron giant want to get hit? No, he's doing it too. And if there are 50 giants on that ledge, they're all doing it. And they all actually have Cover, like the rules say, not Improved Cover.
The 10 "free" AC points assumes nothing to hide behind. You can clearly drop behind cover, a low wall in this case, to give you more cover. What you want to do makes sense.
It is a free action to go prone and a move action to stand from prone. You can easily, within the rules, stand up from prone, attack, & drop prone each round.
If you had shot on the run, you could do it with a tree or other vertical cover.
The only question left is how much cover the wall gives the giant. And as DM, that is totally up to you. Just remember that your players are going to want to do this later.
As DM_Blake said, the whole bending down thing is something anyone using cover is assumed to be doing.
you have the binary of standing or prone to work with, meaning you can stand up (move), attack (Standard), fall prone (free) and repeat. You get to make the call as the DM what kind of better cover he gets from prone behind cover.
And you get to laugh at the party's frustration until they apply the ready an attack tactic. Or if you give the giant improved/total cover they can all attempt stealth checks.
If you want him to have better cover he will then need a move action to get to where he can attack again. What you are talking about is taking a free action to go prone to get better cover. I realize that you want to avoid that, but using the actual game mechanics that is what you have to do.
I would agree, there is specific abilities from classes or feats which would facilitate this, but outside of those (or, say, effects granting a bonus move action to lay down/get up), no, it isn't going to work how you seem to want. Of course, the giant still disproportionately benefits from Cover given their advantage in size/number of squares, in fact so much that I don't even see the need for shenanigans of this sort... particularly if the giant is more than a square back from the edge, I.e. covering more than half their space.
I understand it this way:
You have 10 "free" points of AC (because your AC is 10 + all the other stuff). Those 10 points of "free" AC mean that you're doing all that you can to make yourself hard to hit. Sure, highly dexterous people do it better, but that's a separate part of your AC.
Also, a combat action fills six whole seconds. That's a long time. During those six seconds, you're moving, twisting, bending, ducking, doing whatever you can to A) hit the other guy to the best of your ability and B) avoid being hit.
That said, if a combat situation grants you more AC, you still do the very best you can to use that AC.
To directly answer your question, the bonus to AC the giant gets from his cover already assumes that he is bending into it for maximum effect, doing the best he can to avoid being hit.
In other words, if two giants are on that ledge and one of them is bending/squatting/ducking down to get Improved Cover, then what's wrong with the other giant? Is he a half-wit? Does the moron giant want to get hit? No, he's doing it too. And if there are 50 giants on that ledge, they're all doing it. And they all actually have Cover, like the rules say, not Improved Cover.
I would partially disagree with this. An attack on an armorless, flat-footed opponent has a 10 AC. Such an opponent is not "doing all that [they] can to make [themselves] hard to hit" - though they are still more active and mobile than a grid intersection (AC 5). So the 10 free points is not them naturally doing what they can. In your two giants example, if one were aware of the party and the other not, both would still get cover. Consequently, ducking should improve the cover.
What if he had quick draw?
He could stand (move action), throw a rock (standard action) and drop (free action)
Next round quick draw a rock and repeat.
Would that work?
That should give him total cover, right?
What if the wall is 10 feet high?
Total cover?
By RAW. A prone character and a standing character occupy the exact same space(s) and have identical amounts cover. He would of course get the bonus/penalty to AC against ranged/melee attacks for being prone.
That should give him total cover, right?
By RAW, the extent of how much of the body is seen is what provides cover. Prone will greatly affect the cover of someone. A low wall that reaches a standing person's waist provides cover when they are standing. When that person is lying flat on the ground, unless that person is really round, the cover now is complete.
The vertical positioning of the individual absolutely affects the level of cover. Otherwise, a 30' high wall with no roof (castle wall) would only provide "normal" (+4 AC) cover to a halfling on the other side. Clearly this is not the case.