Ok, I will write an absurd example of how we do such things in our games and hope for the best! (this is probably not RAW)
You try to convince the bouncer to let you stay in the party
Bouncer (NPC) wins initiative
PC declares that will quick talk him into staying
NPC declares that will resist quick talk + flee (he is scared!)
NPC flees with 2 dice (Des + Ath is high (6), but Man + Sub is low (4))
Of your pool of 7 dice (Man 4 Subterfuge 3 whoa! (although it could be animal keen, you know how that guys are...) ) you spend 4 diff 6 but the storyteller tells you that now he is too far to listen to you, so you change your action to 'hang to the chandelier (there is ALWAYS a chandelier in WoD) to close the distance', since you hav a dex of 4 but ath of 1 you have 5 dice max pool, you use 3, you have a +1 diff for changing action
You hang the chandelier and fall more or less gracefully in front of the scared bouncer
Bouncer still doing nothing (as he won the initiative every time all involved characters ends their action (not turn) the initiative goes around), waiting to see what happens.
If the player does nothing (and the bouncer complies) the turn ends, but our intrepid player thinks he has enough and since he is near a piano is going to throw said piano to the bouncer (Str + music anyone? :D) , you have 3 Str and 2 Music but you already spend 3 dices so you have 2 dice left with a +1 diff for changing action (you did declare quick talk after all...) +1 diff for changing action AGAIN (you did declare that you were going to hang the chandelier but nothing else!(that was on purpose, you wanted to save dice for whatever happened after that)) and a +1 diff for second action in your turn (+1 diff every action, so +2 for third, +3 for fourth... powers that give you extra actions negate this)
The Bouncer declares dodge Des + dodge is high (6) and he has 4 dice left with a +1 diff for changing action (he did declare that was going to resist quick talk), the +1 for doing nothing is not applied because he was waiting to know what others declared (you still must remember that it is declared inversely to your initiative).
Our player rethinks this 'I'm not going to put a piano in his head! there is no way I am succeeding in this!' so he changes his action... AGAIN. You know, I rather just punch this dude in the face and the hell with it!
The bouncer then declares that the hell with it also, that he is fit enough to stand it (sta 4) and that he will grappling this stupid dude and maybe kick him out in the next turn.
The bouncer goes first, so his (dex + brawl = 7!) but he has already used 2 dice and the maximum pool from everything he has used (not declared!) is 6 (his Des + ath from the first action) so he only has 4 dice left, with a +1 diff for changing his action from defence from quick-talk and a +1 diff from declaring to dodge and another +1 diff for second action in turn.
Our player thinks 'No way I'm letting this moron grab me!' So he changes his action one more time to enter a competition of grappling (haha! I'm better than this block!) he has Dex 4 Brawl 2 so 6 dice, the last roll was (dex + ath = 5 to hang on the chandelier, since you did not through a piano it doesn't count for the max pool) so, you have a max of 5 dice to roll and you already spent 3! and the diff keeps crumbling up! it is +1 diff to change quick-talk, +1 diff to change throwing said piano (that was a stupid idea after all...), +1 diff AGAIN to change the punch in the face AND a +1 diff for second action in your turn. Man! this is going to be hard...
The Bouncer then changes his action so we now is going to be grappling you PLUS kick in you (NOTE: I'm not sure you can do this because the Grappling rules are very complicated and I don't have books at hand, but bare with me) since it is the same dice pool he still has 4 dice left. Bouncer now has a +1 diff from changing defence from quick talk, +1 diff from changing from dodge the piano, +1 diff from changing grappling contest and +1 diff for second action. The kick has a +1 diff from changing defence from quick-talk, +1 diff from changing from dodge the piano, +1 diff from changing grappling contest and +2 diff for third action (it may not be mathematically possible to do this actions with out extended rules for diffs above 10)
The bouncer comes to you, but has he sees you are going to start talking he gets scared and flees a few paces, with the sound high enough to not to listen to you (maybe this bouncer had enough of certain need-to-talk discipline...), with your agility and grace you cover the distance by hanging onto the chandelier and gracefully falling in front of him, you then faint for the piano, the Bouncer is Genre Savvy to know what is next so he starts to move to do a barrel roll, you got him! now you can cover the few paces separating both of you and punch him in the face! But the Bouncer is just to quick and he grabs you by the wrists, you decide that two can play this game and grab him also but when you are going to grab him... he knees your nuts! (playing dirty are we?)
Some of the examples are exagereting quite a bit to see how the diffs crumble up ad how you can back and forth declare/change actions so the player with more dice gets advantage, how can you 'save' dice to do sudden things at the cost of more diff and how you can if you do not watch out, bury all the advantage you had by changing too many times.
Of course you can see how having the initiative saves you to change actions since you are reacting with a counter-move to whatever people declares, if they want to succeed they need to change and then you change also or maybe you can go for it, you go first, after all...
Having powers that give you extra actions negates that you can divide dice pools (that is a horrible rule but if it didn't you should be stabbing people with a dagger like 12 times in a turn, my ST sometimes admit that we can divide dice pools and sometimes doesn't) but your difficulties don't sky rocket and you have more dice to do this stuff.