My thought was, Noctem's claim about "interrupting actions" may have been intended to mean "prevent the action which triggered them from happening".
Noctem said:As stated before, actions in 5e CANNOT interrupt each other outside of specific examples like the spell Shield. You cannot use a readied action to interrupt another action. You cannot interrupt the Attack Action of a level 5 fighter in between his first and second attack with a readied action unless that readied action specifically states (like shield) that you can. Readied actions as the kind a player can ready by declaring a trigger and action they are going to take in response to that trigger cannot get around the general rules which govern how actions interact.
This is an example in steps:
1. Declare I use Eldritch Blast
2. Use up the action type associated with Casting a Spell and Eldritch Blast (my action)
3. Use any material costs associated to casting the spell I have chosen and perform any other requirements for casting the spell.
4. Finish casting the spell (this is the last step where the spell can be counterspelled because after this you are resolving the effects of having successfully cast the spell)
5. Resolve the effects of the spell (in this case making x number of attacks as described in the spell effect section, say 2 attacks for this example)
Making an Attack
1. Declare target of attack 1
2. Figure out the modifiers if any for the attack
3. Resolve the attack (including damage and effects like forced movement for example)
1 Declare target of attack 2
2. Figure out the modifiers if any for the attack
3. Resolve the attack (including damage and effects like forced movement for example)
Any readied action by an ally would begin now. However, as stated a Shield spell could be used during step 3 of the attacks (Resolve the attack) when you are declared to be hit. This could lead to the attack instead missing, but only because the spell itself allows for this exception to the general rules.
As for your last example, that is correct. You cannot interrupt the action of another creature with a readied action. Immediate Interrupts from 4e no longer exist. Reactions in 5e, outside of very specific examples, work like Immediate Reactions which always happen AFTER their trigger is fully resolved. In this case, the action being used to attack. Note that the creature attacking you moving away would normally provoke an Opportunity Attack which you can perform instead of your readied action. If you have a feat to stop the OA, that's a feat benefit and imo it should be self explanatory that feats are powerful and should remain powerful. But the fact remains that you had to give the attacker a feat to circumvent something you obviously knew would happen to mitigate the strategy you suggested be used to counter the argument I presented. So to you.
Read more: http://www.enworld.org/forum/showthread.php?473235-Warlock-and-Repelling-Blast/page8#ixzz3tP3PVriQ
...can't be dispelled, because the magic exists only for an instant.
Is Magic Missile the only spell which has its attacks resolved simultaneously?
A: Yes.
What does this mean?
1. No other spell currently published has its attacks resolve simultaneously. Arial Black's claim that Eldritch Blast does so is therefore proven incorrect. This also supports JC's earlier tweet that specifies that unless the spell itself says attacks are resolved simultaneously (like Magic Missile does) then they are not by default.
2. This also explains that just because a spell has the instantaneous duration, that its attacks are not resolved simultaneously by default and instead follow the Making an Attack rules like every other spell attack. Arial Black's claim that instantaneous duration means simultaneous attacks is therefore shown to be incorrect.
3. Further questions can be asked if needed.
IF the answer is NO:
1. That means that spell attacks don't have to specify they are resolved simultaneously.
2. It contradicts the earlier answer from JC which explained that a spell has to specify that its attacks are resolved simultaneously and opens up further discussion.
3. A list of spells which do resolve simultaneously beyond Magic Missile can then be tabled and discussed.
4. We can see if all instantaneous duration spells are resolved the same way or only some. To prove or disprove this claim from Arial Black.
5. Further questions can be asked as needed.
@Arial Black
Come on then, this is the question I would like you to ask and the reasons why it's a good question. You can get it posted in only a few moments...
Go ahead, I'll wait.
I think the devs have made it clear if the spell doesn't say simultaneous, it isn't.
I think that's probably true, but there is an interesting question here. Can you move between the blasts of an eldritch blast? You can move between multiple attacks. Are eldritch blast's beams more closely spaced than that?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.