Can you take a readied action in the middle of another character's action?

Question

My question is a generalization of this one: Can you interrupt an action with a Ready Action?

When you ready an action in Pathfinder, the readied player jumps in an instant before the trigger happens (emphasis mine):

Readying an Action

You can ready a standard action, a move action, a swift action, or a free action. To do so, specify the action you will take and the conditions under which you will take it. Then, anytime before your next action, you may take the readied action in response to that condition. The action occurs just before the action that triggers it. If the triggered action is part of another character's activities, you interrupt the other character. Assuming he is still capable of doing so, he continues his actions once you complete your readied action. Your initiative result changes. For the rest of the encounter, your initiative result is the count on which you took the readied action, and you act immediately ahead of the character whose action triggered your readied action.

Are actions atomic (in the computer science sense)?

For each of the different kinds of actions in Pathfinder (Standard, Move, Full-round, Swift, Immediate, Free, and any others you can think of), can one character perform their readied action in the middle of another character's action?

Examples by intuition:

• It makes sense in the question linked above for a readied action to interrupt a move action (e.g. wolf wants to move past me, so I ready myself to sidestep when the wolf comes within 5').
• It makes sense that a person could ready themselves to shoot an arrow when an NPC attempts to light a torch (full-round action).
• It doesn't make sense to interrupt an immediate action (e.g. person says a word, ceace concentrating on a spell, etc.).

Answer 1

No, actions are not atomic. There are numerous cases in the rules where one action can take place in the middle of another, e.g.

• You can take a 5-ft. step (a type of free action) in between two attacks of a full attack (a type of full-round action).

• The Spring Attack feat allows you to use an attack action (a type of standard action) in the middle of your move action (which you may then resume afterwards).

• Enemies’ attacks of opportunity (which does not really use an action, or arguably constitutes its own kind of action) that you provoke by moving interrupt your movement (typically but not strictly via move action), which you may then resume after the attack of opportunity has been resolved.

Readied actions, like immediate actions, interrupt other things going on. They may take place in the middle of another action.

In all of these cases, the interrupted action is put on hold, and the interrupting action is resolved entirely before resuming the interrupted action. These can be nested indefinitely.

Now then, the game does not have strict timing rules for “generic” kinds of interruptions like readied and immediate actions (attacks of opportunity, Spring Attack, etc. do have fairly detailed rules). This is largely left up to the GM. In the case of immediate actions, their timing is defined largely by the individual action’s text, though these are not always entirely clear. For readied actions (and the contingency spell, and others), the triggering condition is defined by the player and adjudicated by the GM, so the precise moment is always kind of questionable. Generally speaking, it’s accepted that while you can have a trigger relating to some roll or outcome, by the time a roll is done, it is too late to prevent the outcome of that roll entirely (instead you react to that outcome the moment it’s in place). If you wish to prevent something, you have to react prior to it being rolled.