So you got interrupted
If you get interrupted while casting a spell, you're in some real trouble. You can run around and melee your enemy while hoping for a Seal of Insight proc, Crusader Striking your way to triple-digit DPS. If you're already locked out of your holy tree, you should probably look into hiding somewhere. Breaking line of sight with your opponents will allow you to minimize your incoming damage while you wait for the interrupt to wear off. If I'm in a Battleground, I like to run behind a wall and then start drinking to regain some mana when I'm locked out.
Instants are always safe
You can't have one of your instant-cast heals interrupted, which means they're ideal for healing when you have a rogue or warrior just waiting to interrupt you. Luckily for holy paladins, Cataclysm significantly boosted the number of instant healing options we have available. Holy Shock is now a potent heal in its own right. Word of Glory is our most powerful single-target heal, although its usage is limited by our holy power generation. Protector of the Innocent, Enlightened Judgements, and Beacon of Light also allow us to spread our instant-casts between more targets than ever before. Technically, Lay on Hands is instant-cast as well, but it has a long cooldown and isn't usable in an Arena environment.
Holy Radiance used to be an option for instant healing, but patch 4.3's redesign leaves it with a cast time. During Infusion of Light procs, Flash of Light also becomes instant-cast. While FoL is taboo in PVE, the heal can see plenty of use in PVP where speed is more important than mana efficiency. All of our Hand Of spells are also instant, which means that it's really only Holy Light and Divine Light that can be interrupted when casting. Exorcism spam can also be dangerous in PVP, as it's tied to the holy tree like every other spell we have.
Sometimes you just have to cast a heal. Whether it's because Infusion of Light isn't always active, Holy Shock has a cooldown, or that Word of Glory requires HP, you will find yourself casting in PVP. If the enemy is ignoring you, then casting a few heals is probably safe. If the enemy is focused on you, however, you need to take some anti-interrupt precautions.
Holy paladins have several tricks for blocking interrupts. Aura Mastery, when activated with Concentration Aura, will allow you to cast freely and without worry for 6 seconds. AM blocks both interrupts like Kick and silences like Strangulate, which makes it a very powerful cooldown for us. Divine Shield and Hand of Protection also allow us to avoid interrupts, although Hand of Protection allows magical interrupts like Strangulate and Counterspell through.
You can also get offensive, using Hammer of Justice to stun a melee class who's training you in order to buy yourself time to heal. I have also often used Divine Protection to give myself a Speed of Light speed boost, allowing myself to get out of melee range of my attacker in order to give me a few seconds of breathing room to heal. Even just running away from your target can give you time for your spells like Holy Shock to come off of cooldown. There's no law that says you have to stand around getting beat up, so move around offensively to create gaps between yourself and your opponents.
Juking that interrupt
The most advanced method of avoiding interrupts is juking. If you're familiar with the idea of a pump fake, then you understand juking. The idea of juking is that you start casting a spell, and then you abruptly stop casting it. If your timing is right, your opponent will try to interrupt your cast, but since you already stopped casting, their interrupt will be wasted. You can then cast all you like until their interrupt is off cooldown again. In order to be successful at juking, you need to have quick reaction times, be aware of your surroundings, and be able to track your enemies' interrupt cooldowns.
The first half of juking involves knowing when your opponent will try to interrupt you. Some enemies are eager to interrupt and will jump on their Kick key early in the cast time. Other players are savvy to your tricks and will delay interrupting until it's absolutely necessary. You need to figure out what style of player you're facing, and you need to know their class. If you juke a rogue's Kick, he can still stun you or Gouge you instead. If you juke a shaman's Wind Shear, the next one could be there in just a few moments. You typically don't want to remain casting for more than half a second during a juke, as if you cast for any longer, even an amateur could interrupt you by then.
The other half of juking involves knowing when it's appropriate. If your friends are going to die soon, juking may be the only way for you to get a spell cast off before they go down. If your partner is at full life, you might not want to risk getting interrupted if you don't have to. Juking is considered to be an advanced caster tactic, so don't worry if you don't pick it up right away.
The Light and How to Swing It: Holy helps holy paladins become the powerful healers we're destined to be. Find out just how masterful mastery healing can be, gear up with the latest gear, and learn how to PVP as a holy paladin.