Always back up files!
Under the installed folder in the path DATA\SCRIPTS\MENUS\ENGLISH are three files; E3.MSC, DEBUG.MSC, AND RELEASE.MSC. Rename RELEASE.MSC to something like RELEASE.MEN, then copy DEBUG.MSC to RELEASE.MSC in this same folder. Now, when the game is started, there will be added menus where many extra options can be set.
If you want the cheats, go to the specific location noted below. When you get to the right place and do the right thing, a message saying "cheat activated" will appear. After it has said this, go to your inventory and open The Book Of Shadows. In it will be a list of all the cheats you have found. Use the control pad to turn them on and off.
I Like Deadside Shotguns-Cathedral of Pain
In the control room beyond the Walk Gad lava, drop down the big shaft containing spinning blades. you will find it as you fall.
Twig Boy-Gad Temple #1
Go all the way through the flambeau maze, and just as you drop back into the big chamber containing Blood Sisters, stationary fire masks, and a big wide ramp leading up to a ziggurat switch.
Play as Deadwing-Playrooms
After going through a pipe, you will come to a hallway filled with doors on each side and a switch on the end. An easy way to know you are in the right hallway is by looking for the nursury. it is beyond one of the doors but is very noticeable as you run down the hall. Go into all the rooms until you come to the room containing a green table. Jump on the green table in the center of the room to find this cheat.
Big Head-Gad Temple #2
Find this cheat in the dark with flambeau doors. it is in the maze on the lower floor. If you just run around through the maze, you will eventually stumble upon it. It's probably the easiest to get because you will probably find it your first time in the maze as you try to find the way out.
Flame Grilled-Temple of Life/Wasteland
At the end of the temple of life their are 5 altars that let you extend your life meter. to get this cheat, run up to the second from right (fourth from left) altar.
Play as Bloodshot-Gad Temple #3
To get this cheat, get into that small bonus room reached from the platform knocked down by the big hammer.
Flame Grilled LeRoi
To cover Shadow Man in flames, follow these steps. You must have at least 100 Cadeaux to pull this off. First, make your way to the very last room in the Temple of Life (it holds the Baton). Next, move to the last small room on the right, and press Action at the altar. Shadow Man will offer your Cadeaux in exchange for an energy upgrade. Press Action again at the altar. Shadow Man will say something about the Loa moving out of that altar. Run out and head to the altar next to the one you were just in. Press Action at the altar. If everything goes well, you should see a "Cheat Activated" message flash accross your screen (in the way Luke's teddy bear does). Go into your inventory and you should see a new item, the Book of Shadows. Select it and your cheat should be there. At the time I found this, I had 24 Dark Souls, and that might make a difference, but I don't think so.
To play as deadwing when you get to the Playrooms go to the room with the pool table. Jump on the pool table and it will activate the cheat and give you the shadowbook.
While working on the first trial of the loa (Temple of Fire area) in the area with two blood falls go and find a room that has 3 voodoo sisters (blue witch things) and a large ramp, kill the voodoo sisters and make your way up the ramp before the button at the top of the ramp look right and you'll see a pathway. Jump on to the railing stuff and jump at the pathway, keep trying and you'll eventually see "Cheat Activated" appear on the screen. Go to your inventory screen and you'll see the Book of Shadows, that is the cheat menu, open it and you know have Stick Boy!
Get Book of Shadows
In the Asylum Playrooms there is a corridor with a number of locked doors. Go to the end of the corridor and throw the switch to unlock the doors. Inside one of the rooms there is one green guy with a shotgun and something which looks like a pool table. If you jump on top of the pool table "Cheat Active" Will flash on the screen and the book of shadows will be in your inventory next to Luke's Teddy Bear.
Play as Duppie
Go to the part in the undercity where you get a teddy bear save.Go fowards and right a bit and kill the gun guy on the slightly higher ground and then go to where he was.Turn right and go foward and you should see a sort of curved pipe.Jump as far as you can and it will probably say(if you did it right)cheat active.It worked with me.You dont have to completely reach the pipe for it to work,just get close to it.I found this out after completing the game and mucking around.
Marrow Gates Mystery Solved
On the level "Marrow Gates" there is a bloodfall to the left. You'll need to climb the bloodfall in order to reach the top. Once at the top, you will find the gate where you need a certain amount of Dark Souls collected. In this gate, you'll need every 120 Dark Souls. Once you get this many, come back to this gate and open it. Then, once inside, you will get the gun called the "Accumulator" the most powerful gun in the game