Most of the secrets are locked out after some kind of point of no return, being them either locked paths that deny backtracking or some specific "incorrect" use of an item on some object or character (as correctly using an item will remove it from your inventory). With this in mind, this guide's sections contain secrets grouped within their playable areas between points of no return, in order of progression.

The secrets list screen isn't shared between saves, which means that starting a new game resets the number of secrets found. With the help of this guide, however, it's all gon be eeeeeezay.

Use your browser's search function to quickly find help on a secret that you need. For experienced players, here's a tip: pressing Escape during cutscenes or quite a few of the animations will skip it, thus shorterning your playthrough.

Found and confirmed secrets counter:

Spoilers ahead!

These secrets are found throughout the whole progress of the story.

On the save/load screen, input part of the Konami code using the keyboard keys found on the bottom left corner of the screen. The code is: Up Up Down Down Left Right Left Right.

                                       

After the kidnapping, every time you open up Otto's cellphone there's a chance that it may get a service signal; he'll comment about it and the secret will unlock. Holding Escape makes this easy to get.
                                       

Press the F1 key 24 times. Yep. Holding it down for a few seconds also works.


Inspect any item(s) from your inventory 100 times in a row. Beware that after you unlock this one, as of now, there's a glitch that makes every inspection of an item end with the same dialogue from this secret, so I'd suggest getting this one later in the game.
                                       

Interact with all light switches in the game. There are 6 in total, and the one on the main menu doesn't count. At the end of each section I'll list all switches available in that area, so make sure to interact with them before proceeding through any points of no return.

  Progression: You just started the game.

In Otto's room at the beginning of the game, interact with the window, then with the soda can.

                                       

Interact with the window, then click the moon a few times (about 15 times) to make it drop.


                                       

When interacting with the computer, try to read all 8 other email messages in the inbox by clicking each of them at least once.

                                       

• Progression: You're on your way to your date... except not.

When trying to bypass the door with the keypad, make sure that you don't remove the flag from the wall before typing in the password (CCCP or CATS).



After your first meeting with E.M.O., while checking out his mainframe, click the ladybug located on the bottom left. You only have two chances to get this one.

                                       

> [1/6] On the hall, near the red flag.
> [2/6] Inside the projector room.

• Progression: You divided by zero and launched a nuke!

Past the E.M.O. room you'll arrive at the forked room with the hole in the floor and the suspicious painting of a safe. To earn this secret, you'll need to walk past the painting at least 26 times. The most consistent way of doing this is going through the entrance to the right (arriving at the room with the filled gap), then back to the middle of the safe room, inside the hole in the floor (craft and use the ladders on it), then back up, rinse and repeat. Thanks to Cavalcadence for finding this!
                                       

During the first conversation with Gusta, the kitchen lady, choose to flirt with her 5 times.


                                       

On the room with the battle tanks and oil tanks, you'll need to stand behind and in front of each of them. Click on certain spots (pictured below) to get to these pesky areas. Thanks to Cavalcadance for finding yet another secret!

                                       

After draining the gap, transverse it 10-50 times, enough to crack a hole on the pipes. To speed things up you can skip the jump animation by pressing Escape, but the "secret found" jingle won't play and the pipe you jump on won't crack until you let the animation play fully at least once.
                                       

In the smelting room (the one with the smelter and anvil), try using Oil on the "AK-47s?" on the wall.

                                       

After getting access to the safe, turn its wheel clockwise 29 times in a row.


                                       

Pass through the hall with the dog without waking him up on the first try. Might be tough, use save files. This secret is awarded after you go through the door, so make sure you know what to do when you reach it.
                                       

After the hall featuring the Hollywood star Mr. Dog, you'll arrive in a room with bricks on the floor and a blinking light that reveals some message on the wall. Quickly inspect the message in the split second it's shown (try spamming right-click) and the secret will unlock.


You'll need to inspect two objects for this secret: In the room with the 42 on the wall, inspect the metal grate on the left, and in the staff room, inspect the wrench before picking it up. Thanks to Cavalcadence for finding the metal grate part.


> [3/6] In the room with the alarm box and the "42" on the wall.
> [4/6] Inside the "secret weapons" room.

• Progression: You've just transversed a radiation filled room.

Next to the fist holding the crowbar at the start of the area, inspect 5 times the shady gap on the wall between the bricks.

                                       

Inside the room with the cardboard girl, after acquiring the duct-tape, keep pulling the rope and pick up about 9 more duct-tape rolls.

                                       

After acquiring the screwdriver and opening up the lock to the men's bathroom, grab the coin but don't use it yet. Go back inside the women's bathroom and try giving it to Dimitri.

                                       

During the interrogation scene, move your (now red) cursor across the boss cat's eyes a few times.

                                       

• Progression: You've escaped from a interrogation room purrfectly.

Interact four times with the graffiti right at the start of the area.


                                       
For this, you'll need to complete the area by fixing the robot, without going near its computer (never triggering the cutscene that pans upwards). For this, you'll need to walk behind the robot to get to the other side (click the spots depicted below), then proceed with the game. Don't forget to get the hose on the starting room! Thanks to Cavalcadance for getting this.
                                       

When interacting with Dr. Funkenstein's computer, after turning on the monitor, click the snake sticker on the top left.

                                       

Interact with the gym's light, then inspect it.


                                       

Right at the start of Swords & Fireballs VII, try to proceed with the game while ignoring its objectives (by not picking up the sword in the pedestal, or ignoring the old man on the camp). Do this 7 times.



While playing Swords & Fireballs VII, you'll be aided by a pink creature called Tecu. After he opens up the boss's door for you, click on him 11 times to send him to a better place.


                                       

Use the hamster from the lab (floor 1) on the toilet (top floor).


                                       

> [5/6] Right at the start, next to the Soviet Stripers poster.
> [6/6] The last one is found at the gym (floor 2), near the boombox.

• Progression: You've succesfully repaired a giant evil commie robot.

After defeating the boss, choose a certain sequence of dialogue to trigger this secret. The choices are: "Who are you?" + "This is a bullshit ending." + "The others?".

                                       

• Progression: You kicked a bossy cat's bottom purrfectly, but the future refused to change.

Click the soldier on the roof.


                                       

Everything points out that the Secret 33 is unlocked when ALL other secrets are uncovered in ONE playthrough, but as of yet it's broken.

Check out the pinned thread for more discussion.

There we go. If this guide has helped you out, don't forget to rate it. And if there is any way that I can make it clearer, do comment.