During a session in the game I am running, we encountered a little initiative problem.
The group was in a very heated discussion (yes, people might call it a fight...) with some individuals bent on harming the PCs. During the fight things progressed to a point, where both parties found themselves on oposite sides of an open door. Both parties decided to lay in wait for the other group to storm through the door and then obliterate them.
The PCs had the initiative and decided to hold their action until someone showed up in aforementioned door. A gammorean NPC decided to storm through the door and to engage and so he did, before the PCs could engage him.
What the PCs wanted was for the NPC to move but shoot him before he could cause any further damage.
In that situation I ruled that the NPC could take his full turn, before the players could act. And that is the heart of the matter. Can you hold your initiative until a prior specified condition occurs and then act, even if it would interrupt the "turn" of another character?
We discussed thi and came to the preliminary agreement, that if the case occurs again the affected parties roll initiative to resolve the situation. The underlying reason reason being, that while you're waiting for a specific event to occurr it doesn't necessarily mean you immedeatly act. But all in all this feels a little clumsy and unelegant, hence please let me know how you'd resolve this thing.