help please - default ui target/focus cast bars

[markhr]

While I'm mostly really happy with the default ui I find the target and focus pve cast bars to be confusing.

Please could someone explain to me:
- why some bars fill from the left and others from the right?
- what the meaning of the different shapes of the cast bars is?
- what cast bar bar shapes/directions are interruptable and what aren't?

Correct me if I'm wrong but for pvp there's only, as far as I can tell, one target/focus cast bar shape and if your interrupt is on cooldown then too bad?