Prime Suspect

Prime Suspect is an Act 2 main quest in Dragon Age II.

Acquisition

This quest is acquired by completing the main quest Finding Home. Speaking with Aveline will prompt her to ask you to keep Emeric quiet, as his relentless investigation into the deaths of young women (which was covered in Act 1 as The First Sacrifice) has caused the city guard to embarrass itself. Speaking to Emeric will continue the quest.

If you accept the quest along a humorous line of conversation: Varric: friendship (+5)

If you accept the quest by beginning the conversation with an aggressive or humorous tone, but end diplomatic: Sebastian: friendship (+5) (Not gotten on PS3 with latest patch) (Not as of PC 1.04). After the second humorous/aggressive response, Sebastian will chime in that "this is a worthy cause." Selecting the diplomatic option at this point accepts the quest and grants Sebastian's approval. This will also net Fenris: friendship (+5). Note that Aveline's responses seem to take precedence, and Sebastian will say nothing if she is in the party.

You will have to accept the quest either way, but if you choose an aggressive response to end the conversation: Varric: rivalry (+5), Sebastian: rivalry (+5)

Walkthrough

Go to the Gallows Courtyard. If you have Fenris in your group, you will enter a dialog scene with him. Choosing the top option nets Fenris: rivalry (+15) ,Choosing the middle option nets Fenris: rivalry (+5) ,and choosing the bottom option nets Fenris: friendship (+5). If Anders is in the group and you select bottom answers, he will argue with Fenris. If Merrill is in your third companion, you will gain Merrill: friendship (+5) if you support the mages.

Speak with Emeric. He will ask you to go to Gascard DuPuis' estate in Hightown to find evidence of his guilt as the murderer of the young women.

If you agree and go to the house, you will find the place full of shades and other enemies. There will be a note on the table in the first room, thanking DuPuis for the last "shipment" (likely one of the female victims) and stating an artifact is on the way as payment (not seen on screen, but presumably the source of all the creatures infesting the mansion). After the battle, go up the stairs and go through the second door from the left. After fighting more shades, a letter from the first enchanter of Starkhaven is on a table in the room. Keep going east and through the only doors that will open. Once you get to a room with stairs, go up them and you will find a letter from Meredith on a table to the right, looking over the room.

When you enter the final room, you will find Gascard DuPuis with an abused and terrified woman named Alessa. He claims to be the one that has been hunting the murderer, although he admits to resorting to blood magic in order to do it. You can either fight him in addition to demons or let him go.

Varric: friendship (+5) if you choose the humorous line, "This I have to hear."
Aveline: rivalry (+5) if you let him go.
Isabela: rivalry (+5) if you let him go. (PC v1.04 may not occur)

Regardless of your choice, there will be a templar named Moira in Emeric's place when you return to the Gallows. She will inform you that Emeric is ready to meet you tonight according to a note you supposedly gave him. As you arrive at the back alley infested with shades and abominations, you discover that Emeric died.

After clearing the area, Moira will arrive. You can tell her you killed Gascard. As he wasn't the culprit, the only storyline impact is the restriction imposed in the later quest, All That Remains.

Otherwise, Moira will tell you there is a dangerous mage about that the Knight-Commander Meredith will want captured and run off. You can either:

Implicate him in Emeric's murder ("Yes. He fooled me.") The templar tells you she will inform the city guard and that Gascard DuPuis will not escape the city alive. This may lead to his execution off-screen by the templars. This restricts your options for completing All That Remains.
Clear him (I don't think so."), which she replies that they will have to continue investigation that Emeric started, considering he was right and died for it.

If you allow DuPuis to live:

Isabela: rivalry (+5)
Aveline: rivalry (+5) (PC v1.04 may not occur)

Rewards

1 (quest completion)
1000 XP (quest completion) and about 400 XP (fight with demons in back alley).
(optional) roughly 1900 more XP if you kill DuPuis and his summoned demons in the estate.

Don't forget to take; Design: Rune of Striking, from a chest in the house.

Bugs

In the room where you first meet Gascard, a rogue in your party who performs Backstab or Vendetta on any shade at the edge of the terrace may appear at the courtyard down below. If this happens, move a long distance away and select the rogue, then select a member of the main group again. The bugged rogue will snap teleport back to the group's location. (If the PC is the one who ends up in the courtyard, then the snap teleport fix will not work. If you try to use it, your party will instead teleport to the courtyard with you, and the only fix you will have is to reload a previous save. You can avoid this by not using the teleport fix and simply finishing the fight with Gascard with the remaining three NPCs and then exiting the mansion. The PC will then reappear with the rest of the party.)
If you let Gascard go, then load a previous save and decide to kill him, Gascard will be impossible to kill in the final showdown with your party.

Trivia

While talking to Gascard DuPuis, if Hawke chooses the sarcastic response, "This I have to hear," Varric says he will bet Hawke twenty silver that DuPuis will reply, "It wasn't me, it was the one-armed man!" This is likely a reference to either the TV series The Fugitive which aired from 1963 to 1967 or the movie The Mask, made in 1994.