Initiative

Initiative Checks

At the start of a battle, each combatant makes an initiative check. An initiative check is a Dexterity check. Each character applies his or her Dexterity modifier to the roll. Characters act in order, counting down from highest result to lowest. In every round that follows, the characters act in the same order (unless a character takes an action that results in his or her initiative changing; see Special Initiative Actions below).
If two or more combatants have the same initiative check result, the combatants who are tied act in order of total initiative modifier (highest first). If there is still a tie, the tied characters should roll again to determine which one of them goes before the other.
Flat-Footed
At the start of a battle, before you have had a chance to act (specifically, before your first turn in the initiative order), you are Flat-Footed. You can’t use your Dexterity bonus to AC (if any) while flat-footed. Barbarians and rogues have the uncanny dodge extraordinary ability, which allows them to avoid losing their Dexterity bonus to AC due to being flat-footed.
A flat-footed character can’t make Attacks of Opportunity.
Inaction
Even if you can’t take actions, you retain your initiative score for the duration of the encounter.

Surprise

When a combat starts, if you are not aware of your opponents and they are aware of you, you’re surprised.

Determining Awareness

Sometimes all the combatants on a side are aware of their opponents, sometimes none are, and sometimes only some of them are. Sometimes a few combatants on each side are aware and the other combatants on each side are unaware.
Determining awareness may call for Perception checks, or other checks.

The Surprise Round

If some but not all of the combatants are aware of their opponents, a surprise round happens before regular rounds begin. Any combatants aware of the opponents can act in the surprise round, so they roll for initiative. In initiative order (highest to lowest), combatants who started the battle aware of their opponents each take a turn during the surprise round. If no one or everyone is surprised, no surprise round occurs.

Unaware Combatants

Combatants who are unaware at the start of battle don’t get to act in the surprise round. Unaware combatants are Flat-Footed because they have not acted yet, so they lose any Dexterity bonus to AC.

Special Initiative Actions

Here are ways to change when you act during combat by altering your place in the initiative order.

Delay

By choosing to delay, you take no action and then act normally on whatever initiative count you decide to act. When you delay, you voluntarily reduce your own initiative result for the rest of the combat. When your new, lower initiative count comes up later in the same round, you can act normally. You can specify this new initiative result or just wait until some time later in the round and act then, thus fixing your new initiative count at that point.
You never get back the time you spend waiting to see what’s going to happen. You can’t, however, interrupt anyone else’s action (as you can with a readied action).
Initiative Consequences of Delaying
Your initiative result becomes the count on which you took the delayed action. If you come to your next action and have not yet performed an action, you don’t get to take a delayed action (though you can delay again).
If you take a delayed action in the next round, before your regular turn comes up, your initiative count rises to that new point in the order of battle, and you do not get your regular action that round.

Ready

The ready action lets you prepare to take an action later, after your turn is over but before your next one has begun. Readying is a standard action. It does not provoke an Attack of Opportunity (though the action that you ready might do so).
Readying an Action
You can ready a standard action, a move action, or a free action. To do so, specify the action you will take and the conditions under which you will take it. Then, any time before your next action, you may take the readied action in response to that condition. The action occurs just before the action that triggers it. If the triggered action is part of another character’s activities, you interrupt the other character. Assuming they are still capable of doing so, they continue their actions once you complete your readied action. Your initiative result changes. For the rest of the encounter, your initiative result is the count on which you took the readied action, and you act immediately ahead of the character whose action triggered your readied action.
You can take a 5-foot step as part of your readied action, but only if you don’t otherwise move any distance during the round.
Initiative Consequences of Readying
Your initiative result becomes the count on which you took the readied action. If you come to your next action and have not yet performed your readied action, you don’t get to take the readied action (though you can ready the same action again). If you take your readied action in the next round, before your regular turn comes up, your initiative count rises to that new point in the order of battle, and you do not get your regular action that round.
Distracting Spellcasters
You can ready an attack against a spellcaster with the trigger "if she starts casting a spell." If you damage the spellcaster, she may lose the spell she was trying to cast (as determined by her Concentration check result).
Readying to Counterspell
You may ready a Counterspell against a spellcaster (often with the trigger "if she starts casting a spell"). In this case, when the spellcaster starts a spell, you get a chance to identify it with a Spellcraft check (DC 15 + spell level). If you do, you can attempt a counterspell to ruin the other spellcaster’s spell. Counterspelling works even if one spell is divine and the other arcane.
Readying a Weapon against a Charge
You can ready certain piercing weapons, setting them to receive charges. A readied weapon of this type deals double damage if you score a hit with it against a charging character.