First: If that was the only part of Counterspell that interacted with spells, your reaction would go after the spell was cast and you would have countered nothing. Reactions do not automatically go before or during the action that triggers them, it has to specified, as is for AoO and Counterspell. Using Counterspell as an example is a bad example. Implying you know when someone is doing something does not equate to knowing when something "begins". Another bad comparison. Identifying a spell requires that the spell is already in course of action. The trigger "i bla bla if target begins casting 'this' spell" would be invalid. To know that the spell is 'this', you would already have gone over the beginning, experience or not.
Second: You ask for "order of resolution" and if would be possible to have a readied action to interrupt a trigger. The answer from the PHB is never, since it states that your readied action goes after a trigger. The trigger can never be interrupted as it needs to come to pass. Now, you can be specific about your trigger, and have some sort of control of when your action actually comes to pass. This control usually breaks the intention of the player tho. "I do not want him to cast a spell" Though luck buddy, by the rules you can't, except for making your target unable to cast a spell via conditions or counter the spell via Counterspell. Reading an action does not make you able to do something that you would not be able to do on your turn as part of one of your actions.
You misunderstand what
counterspell's trigger is.
The trigger is not 'when the spell takes effect', and then it has a special rule which lets you go back in time to just before the spell comes into the world, as you seem to believe.
Spellcasting is a two part process: 'casting', which involves saying the magic words, making the mystic gestures, and/or providing the bat poop (otherwise known as the VSM components). If, and only if, all of these things are completed does the second part begin, which is 'the spell effect'. Until the VSM components are completed, the spell effect does not begin.
Therefore, we can define the VSM components as the trigger,
not the spell effect. We know we can do this because
counterspell demonstrates that we can. This is not a special rule; this is just the trigger. This trigger is as valid for any Readied action as it is for
counterspell. The VSM components are perceivable, and although you cannot have your Readied action occur before its trigger, this trigger is before the spell effect begins.
If left alone, the sequence of events would be: VSM components -> spell effect.
If we were foolish enough to have the
spell effect be the trigger, then the sequence would be: VSM components -> spell effect -> triggers Readied action/
counterspell, and it would be too late and we'd already have a
lightning bolt in the face.
But neither we nor
counterspell are foolish enough to have the 'spell effect' as the trigger. We have the spellcasting process itself, the VSM components, as the trigger.
So the sequence of events is: VSM components -> triggers Readied action/
counterspell -> spell effect (if still possible after Readied action/
counterspell is complete).
The trigger and the readied actions are what you should check for "visibility" in describing what happens. Anyway, all those examples i mentioned i would NOT let pass since i would NOT let them happen, i would ask "what do you want to do" and state clearly that any readied action has to happen after a trigger has come to pass. They still want to shoot the bandit if he opens his mouth? Go ahead, roll immediatly, he has is mouth open to breath. Reaching for the pouch? You are quite confused to when he is about to do it... too much movement It seems like he is already reaching for it... wait... he was scratching. I suppose it was not clear enough in the second part of my previous post.
Police shootings are in the news right now. The police tell a guy to stop, freeze, don't do anything stupid, keep your hands where I can see them, etc. If the dude reaches into his pocket suddenly, he's going to get shot whether it was a gun or his ID.
That's what we're talking about here. You can certainly have a readied action like, 'if he goes for his knife', but that will be interpreted from your own point of view, and you may be wrong.
How it works is like this: you Ready an action to shoot the guy if he goes for his weapon. Fine.
The DM tells you that he is reaching into his jacket quickly. Do you shoot him?
As with any Readied action, when the trigger comes up you have a choice whether or not to execute that Readied action. Is he reaching for his gun? Or his ID? Or maybe a cigarette (
told you those things will kill you!)?
Usually, in our D&D games it's not ambiguous. But if it is you could roll a contest; Insight versus Deception or Sleight of Hand. If you fail, you must choose whether or not to shoot even though you can't be sure.
Which is why most people don't make any sudden moves when covered by a gun! The risk/reward is 'getting my ID out a few seconds quicker' versus 'getting shot'!
This leads those with the gun to shoot, because no sane person would make a sudden move
unless they were going for a weapon.
This is how the word 'perceivable' matters in terms of triggers. We are not claiming any precognition on the part of the Readier. The Readier simply chooses the trigger to be something like, 'if he draws a weapon', 'if he starts casting a spell', 'if he starts to move away from me' etc, as opposed to, 'when he shoots me', 'when he has completed casting and the spell is in effect', 'when he has moved out of reach'.
