In my play by post I have a medium sized humanoid trying to use his body as a shield so somebody else doesn't take a coup de gras on an unconscious goblin.
What bonuses/penalties apply here?
Again:
Players: Human, Goblin, Hobgoblin.
Attempted action: Hobgoblin wants to attack the unconsious goblin
Situation: Human wants to take the hit and is on the same square as the goblin. Would it change if they were on an adjacent square?
Bonus Q: Can first aid be administered from a square away or only the same square?
Bonus Q: only from 1 square away. Two characters can only enter the same square with two really obscure feats.
It is totally a GM call as to whether someone can take a hit for another person. Game mechanics say that when performing a coup-de-gras, they cause an attack of opportunity from everyone within melee threat range. I am unsure about certain feats like Bodyguard, so there may be certain ways of using the AoO to get between the blade and the goblin. I think there may be a combat swap teamwork feat.
Rulewise it doesn't quite work - the Human can provide cover to the Goblin against attacks, but coup de grace does not require an attack roll, so cover is meaningless... the only requirements are that the Hobgoblin be adjacent and the Goblin not be immune to critical hits. Otherwise, only Total Concealment even slows it down (two rounds instead of one).
Playwise... I think I'd allow 'taking the hit' without Bodyguard/In Harm's Way, but it would still be an automatic critical.
Stopping someone from hitting a helpless target is really hard to do short of grappling them. If it's your action, you could move and grapple or bull rush to keep them from closing. If they're already in range and you threaten, there is the option of attempting a disarm or a sunder.
Outside the rules, you can always throw yourself between them as a barrier. But as there is no roll, I'm not sure how to take into account that the knife aiming for the goblin's throat hits your steel shoulder guard instead. It's not like the attacker was aiming for your throat. I'm sure there are several ways to adjudicate it, but since the attacker is being harassed and can't automatically line up a shot, I'd make him roll for it with you as either providing cover or generating some form of Total Defense, but to the goblin instead of you (which I'm sure is NOT the rule). And any hit counts as the crit.
I'm assuming the goblin holds some info or purpose that the hobgoblin really doesn't want coming about? That or he just really knows how to hold a grudge.
You could provide soft cover by standing about the target but as others have stated, coup de gras doesn't require an attack roll, so you're out of luck.
Maybe you could try covering the goblin with your cloak to block line of sight. But I seriously doubt anyone would buy that.
Drag, bull rush, or reposition the helpless character out of range.
Drag, bull rush, or reposition the attacker out of range.
Grapple the attacker.
And so on.
Sadly, there's no "take the hit" maneuver without feats/abilities to that effect.
Other than some of the game mechanic suggestions posted here, there is no rule that covers this.
As a DM, I would like the cinematic flair so I would allow it but make it a hit and automatic critical hit just like the goblin would have taken since the defender is not really avoiding the blow, but putting their body in between the attacker and target.
I would ready an action to attack the hobgoblin if it attacks the downed character. That way, if the hobgoblin attempts to coup de gras, my readied action goes off. If the hobgoblin continues the coup de gras, he provokes an AoO. This way, you can get two attacks off before his attack lands, and I think that's the best you can do.
Also, if allowed, I would step into the downed character's square, and do everything I could (intimidate, trash talk, etc.) to make myself a target and change the hobgoblin's mind.
Perhaps you could create Block as a new combat maneuver that allows you to attempt to prevent an opponent from entering a square, moving past you, preventing attacks upon an ally, deflect a bullrush attempt.
There are classes that have class features to simulate this bodyguard maneuver, like the 3PP Ranger (yojimbo archetype) for the Kaidan setting of Japanese horror (PFRPG)