A warlock and a monk are talking to a NPC. Suddenly, the warlock gets angry and fires an Eldritch Blast at the NPC. The monk recognizes the situation and tries to prevent the warlock from casting.
What can I have them do to resolve their conflicting actions?
(When I say that the monk recognized the situation, I mean just that. The monk and warlock have fought alongside each other for some time, and well, warlock wasn't attacking in a stealthy manner—he yelled "You maggot!", extended his hand forward, and so on—basically what the monk had seen countless times before.)
Eldritch Blast calls for a spell attack roll, for the sake of discussion we can assume it would hit. The monk is probably using his reflexes (so, Dex) to prevent the spell from hitting the victim. We can assume that the victim doesn't move at all.
Idea 1: This just became combat, so rewind and roll initiative. Whatever the outcome, the warlock and the monk have already declared their actions. Depending on who acts first either the warlock fries the NPC, or the monk stops him.
I have a problem with this - the warlock is using his spellcasting abilities, and he could have Dex of 3 and still be a formidable and very quick caster. Using initiative (dex based) is completely nonsensical.
Idea 2: The warlock's spellcasting ability (Cha) vs. Monks Dex. Makes some sense - he is casting, the other one is reacting with his body.
Do the rules specify what to do in this situation? If not, what do you think is reasonable?