Hriston
Dungeon Master of Middle-earth (He/him)
Actually, in most Situations I would start with: I attack and roll initiative. There are situations however where anything other than: Ok, use your readied action makes no sense.
Example: I am hidden and ready an action to snipe the enemy mage as soon as I see him wiggling his hands.
I do assume that I know I am in combat and the mage does not. He thinks he is there speaking to his next victims. I may have my other players roll deception and the one hiding stealth.
So this is your example of a situation that will only make sense if you begin combat with an out-of-initiative readied attack? Let me see if I've got this right. Some of the party are conversing with a wizard, deceiving him into thinking they are his next victims. Meanwhile, another party member is hidden nearby and is planning to begin combat by shooting the wizard. I don't understand why the hidden PC thinks he's in combat since no one is fighting yet. I also don't see how this situation doesn't make sense if the DM simply asks everyone to make an initiative check when the hidden PC decides to shoot the wizard.
But I would not deny them the chance to hit first when they planned that well.
But they do have a chance to hit first, by rolling well on initiative, and it still makes sense if the wizard rolls higher. That would just mean he could take some sort of action in response, like dropping prone to impose disadvantage on your ranged attack.
Even if the enemy wizard has a trident of warning or alert. His benefit will be that he may cast shield and act immediately afterwards.
If I was playing this wizard, I wouldn't consider it an even trade for having an attack made on me without getting to roll initiative.