An Action is a play typically performed during an entity's Action Phase. Actions typically include one or more targets. An Action played against one or more enemies is considered hostile unless otherwise specified and permits a Reaction play from each targeted enemy.
If a hostile Action targets an ally or the entity playing the Action, such a target can only play Reactions if the entity playing the Action is not Controlled or otherwise forced to play the Action by an enemy, and if the Action has the Area Effect keyword and targets (or at least reasonably attempted to target) one or more enemies. In other words, an entity cannot “attack” themselves or an ally specifically for the purpose of playing some advantageous Reaction.
If a target of an Action cannot play a Reaction, or if either the Action or Reaction played lack an opposed roll entry, the Action automatically succeeds and takes effect. Otherwise an opposed roll is performed to see if the Action takes effect.
For more information, see Player's Guide Chapter 3: Scenarios.