Initiative is a property of entities who are in combat. This number which determines the order in which combatants act. Initiative is rolled for in circumstances where it’s not clear which side should go first. In a surprise situation, determination of initiative may be delayed until after the surprise round.
Initiative is only rolled once, and the order established applies for the entire combat. The roll is made on a d6. One player is designated by the DM to roll for the entire party. For each other group of combatants, the DM rolls once each.
Allied combatants whose initiative scores are not separated by the score of an enemy combatant can act in whatever order they please. Some characters have initiative modifiers. These are only relevant when comparing to opposing combatants, not to allied ones.
Note that no matter the initiative order, a given player’s turn can never be split up, i.e. it is not possible to leave some AP unspent, “suspend” your turn so another player can go, then spend more AP. When you end your turn, it’s over.
Suppose Albrecht, Barrow, Cullain, and Digger get in a fight with a group of orcs. The party’s initiative roll is a 2; the orcs’ roll, a 4.
Digger has a +3 initiative modifier, for a personal total of 5. He acts before the orcs.
Cullain has a +2 initiative modifier, for a personal total of 4. He acts simultaneously with the orcs (see below).
Albrecht has no initiative modifier. Barrow has an initiative modifier of +1, but since this is still lower than the orcs, it has no effect. Thus, Albrecht and Barrow can act in either order relative to each other, and this order is not fixed: it can change each round as the two of them see fit. If later an ogre entered combat and got an initiative of 3, Barrow and the ogre would act simultaneously, and Albrecht would go last.
If opposing combatants have the same initiative score after adjustments, then combat for those combatants is considered simultaneous. When there is simultaneous action, all affected combatants are considered to move and attack regardless of enemy actions. For example, if one player strikes an enemy and the enemy is stunned, then that enemy would still get to attack, since the player’s actions and the enemy’s actions are simultaneous. (The enemy would still lose its next turn as normal.)
If necessary, the AP expended by each side can be measured out 1 AP at a time. This can be tracked by referring to the table below (stolen from Alexis Smolensk at the Tao of D&D; I see no need to modify it.)
The left hand column indicates total AP possessed by a combatant. The row at the top is a series of “ticks” subdividing the round. Combatants can spend 1 AP towards an action in each column where a 1 is shown. In the case of a tie, where both combatants can act, the combatant with the next chance to spend AP is the first one to act now; if they would both act at the same time, then their actions are simultaneous, as above.