Not every ability can be interrupted, and not every interrupt can interrupt every interruptible ability. Some mobs are immune to some effects. Generally, if there is a shield icon around the cast bar, your class or specialization can't interrupt that ability.
Players have a variety of abilities that can potentially interrupt a mob, boss, or other player's casting of a spell or ability. They include:
- Interrupts: An interrupt ability interrupts just the spell being cast (and may include locking down the school the spell is in), but doesn't prevent the target from casting spells from other schools. Example: Pummel (unglyphed)
- Silences: A silence will interrupt a spell and prevent any spell from being cast. (Not all abilities are spells, but most classes have some kind of ability that can be prevented with Silence. Frequently this includes taunts.) Boss mobs are frequently immune to silences, but the silencing ability will usually still interrupt a spell if it can be. Example: Silence
- Stuns: Stuns will prevent the target from doing anything, generally for a few seconds at a time. If a mob can be stunned, doing so will interrupt any spells they were casting, but it may not prevent them from casting it immediately after they come out of the stun. Boss mobs are frequently immune to stuns. Example: Kidney Shot
- Disorients: A disorient will make the target walk around disoriented in a small area for a short period of time. Most bosses are immune to disorient abilities, and a disorient ability will not otherwise interrupt their cast, but this works well on non-boss mobs and players. Example: Blind
- Fears: A fear effect will make the target flee, usually in a random direction. Since it causes loss of control, it will interrupt casts and prevent them for its duration. Most bosses are immune to fears, and it will probably not otherwise interrupt their casting, but they do generally work on mobs not immune to fears (undead frequently are) or other players. Example: Fear
There may be others, but these five types cover a lot of the ability types players have that can interrupt spells. Take a gander at your spell book if you're not sure what abilities you have that can potentially interrupt something or someone - look for the keywords listed above.
|Scatter Shot isn't a direct interrupt, but can do so if it disorients a target.|
One of the main differences between actual interrupt abilities (interrupts and silences) and the loss-of-control method of interrupting (fears, stuns, disorients) is that the former will generally put the interrupted ability on cooldown, but the latter won't. So if you have to disorient one of those rare Pandarens to interrupt a heal, watch for them to cast it again as soon as they're no longer disoriented.
Why would you want to interrupt a cast?
- Prevent extra or avoidable damage. Some fights require you to interrupt particular spells just to successfully survive them (for example, Nefarian in Blackwing Descent - the lava phase adds must be interrupted). In other fights, preventing some extra damage will take some of the strain off your group's healers and make the fight easier to win.
- Prevent healing. Some fights include mobs which heal, such as the add packs during the Galakras encounter in Siege of Orgrimmar. Not interrupting the healers can make some fights very difficult if not impossible to complete.
- Prevent loss of control. Some fights have mobs which mind control players if not interrupted. Sometimes the mobs themselves aren't much of a threat, but if they get a hold of one of your players, you might find yourself suddenly dying at the hands of your erstwhile allies.
- Lock down players in PVP. If you PVP frequently, interrupting enemy casters, whether they are doing damage or healing, can make fighting them much easier. In some situations, such as arenas, interrupt abilities can be used very strategically during a fight to make it winnable. In others, it may just help your group capture a flag or a base by helping to lock down a healer or a caster.
|Cast bars can be turned on in the Interface Options under Combat.|
Sometimes there are interruptible abilities that doing so might have a small benefit, but overall not doing so won't make the fight more difficult. For example, in the Siege of Orgrimmar, Sun Tenderheart, one of the three Pandaren that comprise the second boss fight, channels an ability called Sha Sear, which deals damage to the target an anyone standing close to them. The ability is interruptible, but doing so means she just immediately casts it on a new target. Other than to redirect the current target of the ability, there isn't much point in interrupting it. (This is similar to how sometimes even if there's a dispellable debuff in a fight, it's not always worth the mana to dispel it.) If you're not sure, ask, or pop onto WoWhead or Icy Veins and see if it's something you should be paying attention to.
If you're in a group and you're asked to interrupt something, check out what abilities you have to do so. Some classes don't have much that work on bosses (warlocks can especially suffer from this situation depending on specialization), but most damage specializations have something that works on mobs and players, even if it won't help for bosses.
|Silencing Shot is available only to Marksman hunters, but others get Counter Shot.|
If you're not comfortable interrupting, or worried you'll miss the cast, you may need to modify your rotation or hold off using long-cast abilities during the window when the interrupt is likely to come up. Addons like Deadly Boss Mods can help alert you to the timing of boss abilities to take some of the stress off.
Practice on some caster mobs if you need some practice, find a spot on your bars and/or keybinds that you're comfortable hitting it on short notice, and good luck!