The rules says:

Whenever an active enemy unit moves into a space to which a guarding unit has LOS, the unit that is guarding may make an attack targeting the moving enemy unit. Essentially, guarding allows a unit to make an attack in the middle of an opponent’s turn.

A unit on guard can interrupt an enemy unit’s movement at any time to make its attack. When movement is interrupted, pause the move, and determine if the guarding unit has LOS. If there is LOS, an attack can be made. If there is no LOS, movement continues
until it is interrupted again or completed.

So. Can I only interrupt my opponents move action? Or can I intterupt any of the other three actions? First we played that you could, but that made the guard action so strong. You could move, attack and then guard knowing that whatever your opponent did youd get another shot at him before hed get to do anything. Im not sure though since "guarding allows a unit to make an attack in the middle of an opponent’s turn" .

Can I only interrupt when an opponents fig is moving into LOS? Or can I stop him before moving out of LOS? Lets say Im guarding and have a fig in LOS. That fig will move out of my LOS with its first step. Can I interrupt his move action before the fig takes its first step and is out of LOS? Id say yes since "A unit on guard can interrupt an enemy unit’s movement at any time to make its attack" .

"When an active ennemy unit moves into a space to which a guarding unit has LOS, the unit that is guarding may make an attack targetting the moving ennemy unit." page 23

1. You can only interrupt an ennemy movement action. If the ennemy unit perform attack, guard or unit action, you can't attack it during its turn.

2. You can only attack in an hexagone where the enemy move (all the intermediary and the final one). So you can't attack an ennemy unit when in it is still placed in its starting hexe and declare to move out of your LOS.

Thats how I'll rule from now on, but I still feel that it's not completely clear in the rules. From a balancing POV its clear now after a few games.

Thanks for chiming in!