It says you can interrupt the "Action" of a character when you are on hold--for simplicity let's assume the interrupting character was dealt a joker.
Here are a few increasingly unlikely-to-be-legal scenarios to indicate what "Interrupting an action" actually means:
Can a character on-hold interrupt a walking action as a character walks by? Does that attack stop the walk action?
Suppose a character is invisible and turns visible as he attacks a teammate, can the on-hold joker then run up and attack the previously invisible character before he hits? If he kills the previously invisible character, does it still get it's attack (simultaneous)?
Can the on-hold joker wait to see if someone hits then interrupt him before he rolls damage?
Can the on-hold joker see the damage roll and kill the attacker before the character being attacked takes the damage.