During all cutscenes in the Inn, you hold your weapon upside-down and single-wielded.
Obviously a reference to the movie Clue.
You can access Didet the Butler's room without using a skull key by walking up the ladder from Scarlet Drew's room.
First find and talk to all the guests (six)
Second find the weapons (six)
Gather clues from staff members. This involves getting keys from the monsters. There are nine and each give you two clues, either a person, place, or weapon. Once you have spoken to all nine, you talk to Curry (the NPC at the start) and tell him who you think the culprit is, what weapon was used, and where it was done. If you get it right, you have to fight the boss, 1000hp. It's an armoire, and it drops the six weapons from the investigation, and a key for a chest, giving you the costume of whoever was the culprit. (To which you can farm for the color costume you would want.)
All six suspects are references to famous writers of mystery novels such as Edgar Allan Poe and Alfred Hitchcock, as well as Agatha Christie and Angela Whitehill.
If you get the accusation wrong, don't click Go To Room, instead click the (x) and you can re-guess who did it.
It's a good idea to take notes when asking staff. One method is writing down just the names of the authors, weapons or places.
First, concentrate on who did it. Pick the suspects one by one and hit check clues. If Curry says "I think you need to look for more clues" you got the right person. Next, the weapon, Pick the correct suspect and pick the weapons one by one. If Curry says "Something sounds a little off" you got the right weapon. Next is the place, Pick the RIGHT suspect and the RIGHT weapon then pick the locations one by one. If Curry says "this is a good possibility" you have guessed correctly.