No Rest for the Wicked is a companion quest for Isabela in Act 3 of Dragon Age II.
Acquisition
Receive this quest from Isabela from the Hawke Estate. If Isabela ran off with the tome from Act 2, you will first need to visit her at the Hanged Man and be nice to her to get her to rejoin the party. When you return to the Hawke Estate, she should be standing over the writing desk.
BUG: Isabela can be found inside your estate after Isabela's Regret but unable to begin the quest if you have started/completed some other quest. It is best to finish this quest first before beginning any other in Act 3 to prevent this bug. [1]
PREVENTING BUG: At the start of Act 3 go to your estate, a conversation with Merrill will begin, finish the conversation as you like then exit your estate and reenter (NOTE: You should not recruit Isabela back in your party otherwise she will appear in your estate and get bugged), go to the main room where Bodan (The Dwarf) is, he will initiate a quest about your dog's non-stop barking, finish that quest then exit your estate, now you can go to Hanged Man and convince Isabela to rejoin your party, after she agree leave Hanged Man and go to your estate again, now you should see Isabela in the main room and the quest should start normally.
REASON FOR BUG: If Isabela is present in your estate before the dog quest is completed the starting of dog quest glitch Isabela's quest, so make sure you do these quests seperately.
WORKAROUND FOR THIS BUG: (Tested on PC) There will be no Quest Mark for Isabela's "No Rest for the Wicked". Keep resolving other quests or go straight to The Blooming Rose, (the brothel located in Hightown, in the Red Lantern District) in order to continue and finish her quest. From there you will follow Isabela's trail through Hightown, Lowtown, The Docks and will finally head to Castillon's estate.
Walkthrough
Begin the quest by heading towards The Blooming Rose in Hightown at night. Make sure Isabela is in your party. Once there head upstairs to Velasco's room. A cutscene will trigger through dialog. No matter which option you choose, Isabela will be taken away by Velasco's men. Gather your party, then head outside. Follow Isabela's trail which will lead you through Lowtown, and eventually to the Docks to Castillon's Landing. Please note that you can't skip any trail, or skip to the docks automatically. After the cutscene you will have to fight Velasco's men. Once the battle is over loot Velasco for the key to the chest you need to open and grab the documents.
BUG: Be sure you do not loot Velasco until the battle is completely over and the cut scene with Isabela saying "let's look around" plays, or you may not get the key and you will be unable to open the office door; basically, you'll be stuck.
(Optional: for Recipe: Elixir of Heroism pickup the Ambrosia which is in a chest, up the ramp in the northwestern corner of the map. There is also a door to your right, near the entrance. The room is filled with traps. At the end are crates. One of them holds 15 gold.)
Once you head out of the room, Castillon will arrive. Isabela will then negotiate to hand over the Incriminating documents in exchange for Castillon's promise to never come after her again; to which Castillon agrees to. You have the option now to go against Isabela's plans and kill Castillon, or to allow her to go through with this.
Result
In the end, you can decide to let Isabela blackmail Castillon or kill him (for more XP).
Letting Isabela give the documents to Castillon results in: 
Aveline: rivalry (+10), Fenris: rivalry (+10), Sebastian: rivalry (+5).
Killing Castillon results in: Varric: rivalry (+5)
Note
- If you are seeking the Supplier achievement, be sure to grab the Ambrosia in a box in the upstairs office of Castillion's Landing.
- Once you have killed Castillon you cannot loot his corpse.
Trivia
During the opening discussion, Isabela makes a reference to the South Park episode "Underpants Gnomes", saying "Step 1, go to Velasco, step 2, something interesting happens, step 3... profit."