Act 2 => solve all quests before you give the alchemist the 3 cure ingredients (game linearity)
Act 3 => don't enter the catacombs before you solved all quests (you can get the Staff of Power => then the Bishop wants you to unlock the seal. But as soon as you enter, game gets linear again).
Act 4 => Hmm, well you ascend to the next lvl as soon as you have done the main quests - but you can return to each lvl.
