Taking Off-Turn Actions
While the majority of the actions you take during combat will be on your turn, there are a number of actions that can be taken off-turn. Most of these actions occur in response to a specific trigger, but others can be used whenever you'd like (with certain restrictions).
Opportunity Actions
Opportunity Actions are perhaps the most common off-turn action. An Opportunity Action may be used any time you are granted an Opportunity by an opponent, allowing you to attack the opponent who has left itself open and vulnerable. Opportunities are most often granted by a creature moving through the area you Threaten or when a creature attempts to Grab you. The Melee Basic Attack is most commonly used to take Opportunity Actions, but any power with the Opportunity keyword may be used. You are limited to taking a maximum of three Opportunity Actions per round.
You cannot interrupt your own actions in order to take an Opportunity Action (i.e. if you are in the middle of performing an action when the opportunity is granted, you cannot take the opportunity). For Example, when in Cull the Weak Stance you gain an Opportunity when an adjacent target is knocked Prone. If you decide to use Thug's Whomp on an opponent, and succeed in your attack, you knock the target Prone. Since you are in the middle of your action and causing your opponent to be knocked prone, you cannot take the opportunity for the creature being knocked prone.
Reactive & Interruptive Actions
Reactive Actions and Interruptive Actions are actions that occur in response to a certain event, known as a Trigger. Reactive Actions occur after the triggering event completes, such as after you are hit with an attack or after you fail a saving throw. Interruptive Actions interrupt the triggering event, allowing you to interrupt an incoming attack and deflect it or interpose yourself as the target of an attack that is meant for your ally. A Reactive or Interruptive Action can be taken at any time (either off-turn or on-turn), provided that the Trigger condition is met. Some Reactive and Interruptive Actions have a Recovery Time, which adds to your current Recovery (effectively Delaying your next turn). You are limited to using one Reactive or Interruptive Action per round. You may not use Interruptive or Reactive actions during your turn.
Immediate Actions
Immediate Actions can occur during any segment, either on your turn or at the end of the segment, after all creatures acting that segment have taken their turn. If more than one creature is taking an Immediate Action in the same segment, then actions are performed according to Initiative Order. Some Immediate Actions have a Recovery Time, which adds to your current Recovery (effectively Delaying your next turn). Unless the action has a Recovery Time, an Immediate Action taken on your turn my be directly followed by another action. You are limited to using one Immediate Action per round.
Free Actions
Free Actions can occur at any time, either on you turn or off-turn. Some Free Actions may be taken at any time, while others occur in response to a specific Trigger (like Reactive and Interruptive Actions). Free Actions are generally unlimited, though as a rule of thumb, most GMs limit to one Free Action per segment. Even though Free Actions aren't limited in and of themselves, sometimes the powers and abilities that produce the actions are limited in the frequency of their use.
No Action
A very few abilities require No Action to use. These abilities can be used at any time, even when normally prevented from taking actions (such as when Asleep or Unconscious). Typically No Action abilities occur in response to a specific Trigger (like Reactive and Interruptive Actions) or signify a continuous effect that is sustained even when Incapacitated.
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