The rules for falling do not address timing or actions at all. They only specify damage taken.
However, the general rules for the game are that things are atomic—you do one action, you do another, and you cannot mix them or interleave them. There are exceptions—Spring Attack, 5-foot-steps, etc.—but they are exceptional. Without some explicit exception, I would default to assuming that you must finish falling—more on what that means in a bit—before you can do something else.
Therefore, if, at any point in time, there is no longer ground beneath you, you fall immediately. You are not in control and cannot act normally. If it was your turn, that pauses until the fall completes—details still forthcoming—and it is effectively no longer your turn. This is similar to the situation with attacks of opportunity, which interrupt an action or turn to allow a different creature to act (and attack).
Some actions, principally immediate actions, can be taken out-of-turn and therefore always can occur during a fall. Feather fall is even specifically designed to be used just as you begin to fall. Likewise, if you have some relevant ability to act in the middle of movement—maybe Spring Attack if this was an intentional jump off a cliff perhaps—I would expect most DMs to allow it to apply, though the rules are murky about that. You could also presumably ready an action to take place after you’ve fallen a certain distance or when you’re a certain distance above the ground, but you would have to ready the action prior to falling in my estimation.
Dungeon Master’s Guide II specifies that on the first round of a fall from a very great height, a character falls at most 670 ft. So if you are higher than that, the fall “completes” for the turn, and you act after that.
Thereafter, each round is another 1,150 ft. Round-by-round durations in 3.5e are somewhat odd, but very clear: anything that is supposed to occur X rounds after some point, occurs X rounds later when you reach that same initiative count. Initiative does not change during a given turn, so your fall began on the relevant initiative count, and you continue to fall as soon as the initiative reaches that point again—that is, before any creature acts on that initiative count.
These numbers (670 ft and 1,150 ft) are a fair bit larger than dlras2 had calculated using the relevant real-world physics,1 but then maybe the acceleration due to gravity is larger in D&D than on Earth.
There are no restrictions present in the rules on how you may spend your actions if your turn comes up in the middle of free-fall (that is, after dropping 670 ft in the first round, or after dropping 1,150 ft on any round thereafter). Presumably, if you only have a land-based movement speed, you would not be able to actually move, but you would get a move action, as well as swift and standard actions, as usual. Because of the abstractions present in the rules, this means that for shorter falls, if you fall during your own move action you can land and still make your Standard action afterwards. In theory, maybe some or all of that is actually going on during the fall but the game does not model that.
Personally, if this was any thing like a major part of a campaign I was running, I would houserule some way to move as a move action while in free fall—even without any kind of equipment you can aim yourself in a long fall, though I’d have to do some research on how much.
- See the comments for dlras2’s calculation. Using real-world physics and a lot of estimation, and assuming you take 20d6 damage, i.e. 200 ft., to be the point at which you hit terminal velocity, you would move 470 ft. in the first round and 660 ft. per round thereafter.
As the spell description states, GMs can be a bit more liberal with how they interpret the results of Grease. A save would be keeping more in line with the spell, but a climb check wouldn't be out of the question.
The reflex save can be thought of as reacting to the hand/footholds suddenly becoming slippery. They fail, and they lose their grip and plummet as they slide off the wall. Success means they hold on (immobile).
The climb check as a direct result of the spell could be if the target already has a solid grip, which is unaffected by the magic. Movement (which maybe the target was in the middle of) would require the check to see if they can stay on the wall, with at least the -5 penalty for a slippery surface (maybe even -10, considering the wall as a creature who suddenly got the +10 to their Escape Artist).
Either way, further movement would require a climb check at penalty. As per the spell description, going with the reflex save is the more canonical answer.
Best Answer
There's no official rule for this, but it's a common enough house rule. In D&D 3e (and AD&D) I allow them to make whatever check is situationally sensible to try to catch them, and then require a Strength check for them to not be pulled over as well. Some DMs seem to feel like it is being "too soft" on the players to give them a second chance, but in my experience, giving them a chance to catch their comrade with a house rule is not unbalancing at all. Because it's not unbalancing to give them a chance, the actual DCs and checks you use don't really matter for balance purposes either.
The implications of this house rule are actually rather interesting. It feels like giving the PCs a break, but it actually just ups the ante on the trap. Instead of having one character fall in on a bad roll, that bad roll triggers a double-or-nothing gamble: neither PC falls in, or both fall in. It's actually rougher on the PCs because the stakes are higher, and it makes it more likely that there won't be anyone at the top who can help them out of the pit.
Hilarity ensues when a third PC tries to stop the second one from being pulled in, and so on. I've had an entire party rope themselves together, forget to tie the rope to anything other than a PC, and all of them fall down a chute to a nastier level of the dungeon due to bad rolls.