Here's how I see readied actions, generally.
First, they either go just before triggering action or they interrupt it (counterspell). In either case, to avoid the following weird sequence, the original actor MUST continue his action, or at least attempt to do so and use up the action.
The problem:
A. I attack B (throwing my spear)
B. No, wait, my readied action was to duck behind the wall if you did that.
A. OKay, then, you are behind the wall. Since I haven't yet actually acted, I will move first, and then attack you.
B, No, wait. If you are not attacking right away (by taking a move before attacking), then my readied action does not happen and I'm back in front of the wall and I'm STILL ready to move should you attack me.
Aaaarrrrrggghhhh.......
Keep in mind that while this might make a readied action seem too powerful by circumventing the normal game mechanic to dodge an attack, you do so at a cost - the cost being the potential of losing an action all together.
I see it happening one of two ways:
1.
A: Readies his move action (as above).
DM: (Secretly) Rolls a Sense Motive check maybe, perhaps DC 10 (I just made that up), for B to notice something's up. B makes the check.
DM: Okay, B, you notice that A is ready and waiting for something.
B: Okay, then, I assume he's waiting for me to attack him again with my spear of returning, so I'll attack someone else instead, and A won't get to take his readied action if I'm right.
A: Drat - I get no action now and B just hit C (hard)!!
or
2.
A: Readies his move action (as above).
DM: (Secretly) Rolls a Sense Motive check maybe, perhaps DC 10 (I just made that up), for B to notice something's up. B makes the check.
DM: Okay, B, you notice that A is ready and waiting for something.
B: Well, I think he's getting ready to interrupt my spellcasting buddy, "D," so I'll attack A with my spear.
A: All right! My readied action kicks in, and I duck out of the way behind the wall.
DM: Sorry, B, but A ducked out of the way. I'll generously allow you to retarget your spear or to not throw it, but either way you are using up your attack action.
That's how I'd run it.