There are a few bosses where your role is entirely dependent on raid comp, so take this post as an anecdote, not rules about spriests.
Beth could have you dotting up the drone while AOE'ing the spiderlings, but if your group has plenty of AOE capable classes, or too many single target classes, that may get switched around. On Ryolith, DoTs on the legs can be a pain, so you're likely to be AOE's down fragments and burning sparks until phase 2. On Shannox, you want to multidot everything (3 boss targets) while looking out for traps. You are almost guaranteed to be the flier for Alysrazor since we don't have an interrupt as Shadow, so search for those how-to videos on Youtube. There's also a ton of threads on how to maximize DPS while flying on this forum, so browse a bit if that turns out to be correct. You also have a high chance of being in the crystal rotation for Baleroc just because of numbers (requires 4 people on 10 man, usually done by DPS), so be aware of the debuff when you have it sub 25% so you don't oneshot yourself with SW: D. On Staghelm, the adds during cat are best killed by melee, but you'll probably be on orb stack duty, so be ready to run around for that like you did on Baleroc. It's also likely that you'll be expected to use both Hymns to help the healers at some point, so be ready for that on Staghelm. For Ragnaros, you're likely to be chosen for trap duty because you have levitate, so get the minor glyph for it and test out your timing to make sure you can time the cast to minimize falling time without actually dying.
All that is assuming that you know the basic mechanics of each fight. Those are just the roles you're likely to play besides the expectations of DPSing.
Originally Posted by Purpleisbetter
Power Torrent, Volcano and Theralion proc'ed, dots just refreshed. Everyone dies. Just a tank and a boss with around 200k hp. Everyone in vt yells "omg we failed omg omg" and you "don't worry amigos, my dots are steamrolling!". Boss dies while you'd say "Enjoy your loots" with a lot of purple awesomness spilling thru your voice. Just happend yesterday.
Seriously, i thought i'd reroll warlock for 4.2, but that was the sign that i'm purple inside and i can't reroll. never.