an immediate action can be performed at any time—even if it’s not your turn.
(Actions in Combat → Immediate Actions)
This is literally all we get, “any time.” The only way I feel this can be understood is as what it says—any time. This is problematic because the game doesn’t define time very deeply—there’s rounds and turns and actions which break up time in specific ways, but this decidedly doesn’t cover “any time.” It would be great if the game did—it would allow us to just point to a rules quote and say “here’s the answer,” to this and myriad other, related questions—but it does not. What we are left with is a tension between the discrete chunks of time that we do have, and the reality of taking various physical actions that we know take some amount of time, which we expect the game to emulate barring an explicit statement to the contrary. To me, it is clear that time exists and is experienced by characters in a way that is similar to reality, and that the abstraction of rounds, turns, and actions is just that, an abstraction, and “any time” isn’t limited to it—but others disagree.
Nonetheless, even if we don’t tie things tightly to the rounds, turns, actions structure, “any time” does still imply some limitations.
For example, when attacking, the rules for attack rolls specify that “If your result equals or beats the target’s Armor Class, you hit and deal damage.” There is no time between an attack roll being made successfully, and damage being dealt. So you cannot use an immediate action in response to an attack roll and get your action in before the damage.
On the other hand, realistically, attacks take non-zero time (a sword takes time to swing, an arrow takes time to fly), so you can use an immediate action in response to someone starting to attack. You just have to do so before they roll if you want to get your action in before damage is dealt. (The rules are not clear on whether or not attackers have to declare who they’re attacking, so you may have to use your action without knowing for sure that you are the target.)
This is the only guidance the rules afford to us, and so this is going to be how I adjudicate the answers to any questions about immediate actions. Though other limitations may well be reasonable houserules, as “any time” is quite powerful and also quite difficult to manage as a GM, I don’t find it plausible to tie “any time” to turn/round/action chunks. For instance, that would fly in the face of the stated purpose of options like emergency force sphere, which says “Normally this spell is used to buy time for dealing with avalanches, floods, and rock-slides,” which it could not do if the spell were unable to be used after such a thing had started but before it buried a character, even if that was one action. On the other hand, this example also does well demonstrating the possible reasons for such a houserule—emergency force sphere is a preposterously overpowered spell. My preference is to simply ban emergency force sphere instead, however.
Anyway, on to your individual questions, answered with this understanding of time within the game:
- A single melee attack from a creature who starts adjacent to me on the beginning of its turn.
As long as you use it before it has made its attack roll, yes, you can interrupt that. The creature will use its attack, but miss.
- All iterative melee attacks from the same creature.
First of all, remember that creatures can take their five-foot step during a full-attack. If a creature didn’t have to move to begin their full-attack, and you take an immediate action to 5-ft step away, they can simply 5-ft step towards you and keep attacking. One attack is wasted, but the others are not.
Likewise, obviously, if you step into another square they can threaten, they can keep attacking you; the fact that you moved during a previous attack has no effect on the remaining ones. In fact, since the rules don’t specify that the creature has to declare where they’re attacking when they begin to do so, it may be that they can even still attack you with the first attack in this case. (The rules are unclear about this.)
Finally, even if they cannot take a 5-ft step, creatures can choose to make a full-attack or not after seeing the results of their first attack. If you immediate-action yourself away from a creature and it misses its first attack, and it has no other targets, it will presumably treat that single attack as its standard action, and then use its move action accordingly.
- A single melee attack from a creature who takes a regular Move action on its turn to move adjacent to me.
If you use Psychoportive Talent to move while the creature is moving, it can change its movement accordingly. Like attacks, each square of movement is an individual decision for the creature: it is never “locked in” to going to a particular square next. Even when running or charging, which prevents a turn, the creature can choose to simply stop if it wishes.
If you wait until the creature has arrived at its destination and begun to attack, this is the same as 1.
- An attack from a creature made as part of a charge on its turn.
No difference from 3.
- All iterative attacks from the same charging creature with Pounce.
Same as 3 and 4 except that if you wait until it arrives before using Psychoportive Talent, this behaves like 2 rather than 1, except of course that they are committed to a full-attack and cannot take a 5-ft step as they have already moved (the charge). This may mean their options for “salvaging” the turn are highly limited.
- A projectile ranged attack (i.e. crossbow bolt) targeting me.
- Same as above, and I am able to Psychoport behind an obstacle to break LoS.
Projectiles have finite speed and therefore non-zero travel time. Here, even if the creature doesn’t have to “declare” a target, you’re still looking at a projectile in flight and can see its direction (though honestly I feel much the same way about a sword swing anyway). So you can use an immediate action after seeing where the projectile is going, and where it’s going cannot be changed after that point. Whether or not you break line of sight with your movement is irrelevant, because the projectile was already going to miss no matter where you went.
- A Ray attack targeting me (same consideration as above).
This is unclear; we do not have strict rules for whether or not a ray is instantaneous (or as good as, e.g. a laser) or if it has a perceptible travel time. If there is time in which you can act, then you can avoid it. If there isn’t, then to avoid it you have to get out of range or behind cover—the choice of targeting would be the last step in casting the spell and too late to avoid if the actual travel time of the ray is zero.
- A Line/Cone/Burst/Spread spell effect, when I am able to Psychoport out of its area of effect.
Similar to the ray, depends on the travel time of the effect, which isn’t defined. You may need to move before it has been targeted, meaning you not only have to get out of its area, but out of any area within its range. If you can do that, though, you can avoid it.
As you can see, this makes Psychoportive Talent extremely powerful, foiling a wide variety of effects, costing opponents significant resources, and at almost no cost to you. D&D 3.5e had a similar feature, abrupt jaunt, which was a 10-ft. teleport 3+Int times a day—this was widely and strongly recommended for banning. I feel likewise about Psychoportive Talent (and I’ve worked for Dreamscarred Press, though after the trait was published). I do not consider the problems with Psychoportive Talent or emergency force sphere to justify nerfing all immediate actions, however, which is what tying immediate actions to the game’s abstract time-chunks does. It is easier, in my view, to ban the problematic cases, than it is to develop and institute and enforce timing rules on immediate actions.