Hidden Extra 1:
When you start in the Asylum, the extra is through the window in the next room over. Go down the hall and get the gun. . Go back and shoot out the window. Now you can climb through and get it.
Hidden Extra 2:
In the Asylum, where you get on the elevator, there are two bathrooms. Search them and open the stall doors.
Hidden Extra 3:
After you electrocute the man on the table, head down the stairs. There will be an open door on your right.
Hidden Extra 4:
Get Sam. Enter the graveyard. After the first group of Deadites, there is a break in the fence. Go into the water, and far in the back is the extra.
Hidden Extra 5:
After the graveyard save point, you hear the loud footsteps before you learn to possess Sam. On your left is a stream. Go into it and back.
Hidden Extra 6:
After you turn into Sam for the second time, go down into the cave. Go into the stream. The current will take you there. (Sam will get killed on spikes.)
Hidden Extra 7:
In the mine level, on the platform where the second spirit is found, go to the right (away from the egg).
Hidden Extra 8:
In the Woods level, you reach a clearing where you'll be attacked by Deadite Dogs. After you beat them, you'll see a bridge that is up on a ledge. To the right of it is the Hidden Extra. Continue through the level until you reach an area where you ride a Deadite using Sam. Fight your way back towards the clearing. Before you reach it, there will be two small cliffs you can climb up. Get on the ledge, shoot the hole at the top of the bridge with the Harpoon Gun and it will pull the bridge down. Cross to find the extra.
Hidden Extra 9:
In the mining area, defeat the flying Deadites. To your left will be a building and two crates. Use the crates to get on top of the building. You can now target a bridge with the Harpoon Gun. Once it is down, cross and shoot the mining car. It will be pulled out of the way, revealing the extra.
Hidden Extra 10:
At the start of the Forsaken Mine level, there will be a mining cart to your right. Shoot it with the Harpoon Gun and pull it out of the way to reveal the extra.
Hidden Extra 11:
When going for the second spirit in the Forsaken Mine level, turn to the left instead of going to the egg to find the extra.
Hidden Extra 12:
In the Shipyard level, after the save point, you will encounter a group of fat Deadites. After killing them, enter the blue crate on the left.
Hidden Extra 13:
In the Shipyard level, after being ambushed in the warehouse, climb the crates and go through the door. Go down the walkway. On your left there will be a sign that reads "No Smoking". Go down from that and turn left after the building. Go down the walkway to find the extra.
Hidden Extra 14:
On your way to getting the second spirit down the stairs, kill the big Deadite. Go back to before where you get the spirit and down by the semi containers to find the extra. Look out for an ambush.
Hidden Extra 15:
Posses Sam in the swamp. Around the swap are piles of trash. Ride the Deadite out of the fence and smash the trash on your left.
Hidden Extra 16:
Open the gate after defeating the big Deadite as Ash. Go into the swamp. Go under the bridge, and past the ladder. On the right is a cave which you can open with the rocket launcher
Hidden Extra 17:
When you enter the town, go down the main street. On the right is an alley. Go down it.
Hidden Extra 18:
In the area where Sam needs to open the warehouse door, walk to the back of the building
Hidden Extra 19:
At the start of Alternate Demontion, turn right and follow the path.
Hidden Extra 20:
When you finish the arena, go around the walkway.
Thanks to Revolution readers timetomine, daylightcannon, and penciltoankle!