Hello,
First time poster long time reader. Hope to write it as clear as possible.
EAREE
EBAEE
EMEEE
E = empty square
A = adjacent character
R = character medium sized with reach weapon
M = moving character toward B
B = empty square and final destination of M
Does M have cover and, therefore, not AoO is made during his move action?
A|B
A and B = Hostile characters in a corridor
| = door splitting the 1,5 f corridor
Can A and B open and close the door each round despite their adjacency to the door?
First question:
No, "M" does not have cover because "R" is using the ranged attack rules to determine cover.
To determine whether your target has cover from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target’s square passes through a square or border that blocks line of effect or provides cover, or through a square occupied by a creature, the target has cover (+4 to AC).
When making a melee attack against an adjacent target, your target has cover if any line from any corner of your square to the target’s square goes through a wall (including a low wall). When making a melee attack against a target that isn’t adjacent to you (such as with a reach weapon), use the rules for determining cover from ranged attacks.
So, "R" selects his bottom-left corner, then draws a line from that corner to each of "M"'s 4 corners. Since none of those lines pass through an ally (or other obstacle) then "M" does not have cover relative to "R".
"R" may make an AoO against "M".
Second question:
If I understand your question correctly, you are asking if being adjacent to a door in some way impedes your ability to open it? No, it does not.
Both A and B can open or close the door. Of course, the GM may have to make a judgement call and ad-hoc some rules if one or the other is trying to impede the door being opened or closed.
1) Ok, I wasn't sure if two diagonally adjacent characters were to be considered as two horizontally adjacent characters for determining cover. I assumed that both A had to be considered as a continous covering effect.
2) I was interested only in ruling. Thank you.