Interrupt

To interrupt an action in the game is to prevent it from being completed. Every skill in the game has an activation time during which the character is preparing the spell. If a character is interrupted during skill activation, the skill is interrupted. This has three results:
 * 1) The skill will not be executed obviously.
 * 2) The cost of the spell (if it costs energy or adrenaline) is lost to the character.
 * 3) The character must wait until the recharge time for that skill passes by before using it again.

There are many skills in the game that allow a player to interrupt a character's actions:
 * Elementalist: Lightning Javelin, Maelstrom (spells only)
 * Mesmer: Clumsiness (attacks only), Cry of Frustration, Guilt (offensive spells only), Leech Signet, Power Leak (spells only), Power Spike (spells only), Power Drain (spells only) and Shame (Spells cast on allies only)
 * Monk: N/A
 * Necromancer: Mark of Subversion (Spells cast on allies only), Spinal Shivers
 * Ranger: Choking Gas (spells only), Distracting Shot, Concussion Shot (spells only), Punishing Shot and Savage Shot
 * Warrior: Distracting Blow, Disrupting Chop, Savage Slash, Dwarven Battle Stance

In addition, any skill that knocks down an enemy (like Hammer Bash or Bane Signet) will interrupt their current action.

Tip: To effectively use interrupting attacks, keep an eye on the enemy's bar in the center of the top of the screen. When the enemy starts using a skill, the icon of that skill will appear under the bar. If it's a skill you want to interrupt (and can interrupt), go for it!

Easily Interrupted
Some skills in the game are labeled as easily interrupted. This means that this skill will be interrupted if any attack hits the character, not just an interrupting attack like the ones listed above. All Ranger traps are easily interrupted. Other examples of easily interrupted skills include Crude Swing and Precision Shot.

Being Dazed causes all of your spells to be easily interrupted.