Hey, I just thought I'd comment on some more aspects of a 19 lock. Drayner basically summarized how I tend to play (though I tend to be a bit more defensive) and like him, my playstyle basically depends on my mood.
Against a large group of enemies, assuming I have support, I usually strafe (E and Q) around the bulk of their army, all the while dotting them up. I usually dot any healers first, followed by rogues to prevent stealth, then hunters, then other classes. If I have a good healer on my team or none of my opponents attack me, I usually focus on the enemy healer. Hopefully, my allies will do the same, but if not, I find that locks can keep a healer at bay long enough to allow the lock's allies to take out the bulk of the healer's team. Arcane Torrent provides an additional tool for shutting down healers, and Fear, even when met with an insignia, will keep any single target out of a fight for an extended period of time.
Against one enemy, such as in duels, the technique is pretty strait forward. Against any class, the basic strategy is to cast both dots, then Fear, then follow up with either Drain Life if you've taken some hits, or Immolate, Shadowbolt, or Searing Pain if you are near full health. A good opponent will have an Insignia equipped, and will probably use it as soon as you Fear. Be ready for this.
Also: some classes have abilities that will interrupt your spellcasting and silence spells of that school, such as a Rogue's Kick. If you get caught with a kick while casting a Shadowbolt, Fear, or Drain Life, all of your shadow spells, including Fear and your DoTs, will go onto cooldown. This is BAD. A warlock without shadow spells is a warlock with almost no potency. Fearing an enemy with such a move before they reach you is one way to prevent this. Another trick that I've found to be effective is to start casting fear when your interruptin' opponent is near melee range, then to cancel it half way through. Often enough, your enemy will was their interrupt which you aren't even casting, which will allow you to follow up with another, uninterrupted Fear.
I have a question for some of you other locks out there: which DoT do you cast first? Corruption or Curse of Agony?
