I don't really get what you mean. Can you elaborate?
In most RPG's, whenever you enter a situation where you have discrete rounds (whatever term they're given in that RPG), the actions of one actor (either PC or NPC) are initiated and resolved during that actor's turn in that round. Outside of that turn, it's very difficult for an actor to initiate any actions.
Rolling it back to D&D for a second. In 3.5 D&D, you roll initiative and get your turns in combat. During your turn, you take your actions (move and standard) and you're done. Unless something draws an AOO from you, you don't get to initiate any actions until the next round and your turn comes up.
In 4e, that's not necessarily true. For one, it's easy for one character to grant you extra actions - the Leader role takes this as its primary function (other than possibly healing), but other classes can do it as well. Additionally, a number of the powers are interrupt style effects - things that you can choose to initiate on someone else's turn, frequently requiring very little to trigger that action.
Take something like "Ripost Strike", a 1st level Rogue At Will. If you hit with this attack, you deal damage and if the target of your attack attacks you, you get a free shot at it. Not exactly a hard thing to do. Additionally, there are a number of magic items and whatnot that act in a similar fashion. And, there are powers that trigger off of ally actions, many of which are completely under the control of the players at the table.
As I said in before, it's not terribly difficult for a character to be acting two, three or even four times in a given round, all on different initiative counts. It's not that this was never seen in earlier editions, but, never to this scale. Sure, you might get an extra shot with something like Trip and hitting them when they stand up, but, in 4e, it's baked right into the classes.