Get a bronze or better on the Rare -Or Well Done challenge on Behead The Undead to unlock Carrion Carcass.
Successfully complete story mode on the normal or hard difficulty setting to unlock Corporal Hart.
Successfully complete the Amateur League with golds to unlock Dr. Peabody.
Successfully complete the Behind The Undead challenge (the first one) with bronze, silver, or gold to unlock the Zombie Monkey.
Successfully complete the Cats Out Of The Bag challenge with bronze, silver, or gold.
Go to arcade mode and choose any game type except for "Mapmaking". Choose "Disco", then select any character. Find the drum, microphone, and guitar. Press X near the drums. Each time you press X, a different drum will sound. Go to microphone and it will screech. Have a friend do one while you do the other. Note: The guitar never makes a noise or sound.
In the factory where the TimeSplitters are being incubated, the giant energy sphere powering their growth can be taken out on the top level by flicking both the switches on either side of the ball. This will lower its defense. Then, take the lift down to the floor below and throw a plasma grenade (found next to the lift) at it, causing it to explode.
If one character under control moves too far ahead of the other one, the one who was left behind will "teleport" a few feet ahead of the first character, often resulting in strange camera angles.
When you shoot off a zombie's head, look very closely at its neck (preferably with a sniper rifle of some sort). You will see a miniature head where the normal-sized one cracked off.
Equip a spawn-an-attack INSETIK with a baseball bat in Mapmaker. All it does is walk a few feet forward and knock itself in the head with the bat. Even if you go up to it, it will not attack.
Some story AI characters in Mapmaker have the ability to fire some weapons like machine guns. The most noticeable way to make this happen is to set a Cropolite with a Flare Gun. Although it takes about three seconds to load a Flare Gun, some Story AI characters still fire it rapidly without ever worrying about reloading.
Note: This may or may not work with any other character besides Berserker Splitter. Go to Mapmaker and set up a Berserker Splitter with a key. When it does its camouflage cloaking and reappearing, it will accidentally drop the key. It is possible the game does not treat the cloaked Splitter as a solid character and causes the key to fall.
In these challenges you are fighting zombies. Keep moving and you will not have a problem. Make sure to stay out of corners and get used to your surroundings so that you can move and not get trapped between the zombies. It is possible to get at least 75,000 points on every one of the three by using this technique.
Enter the prison cell on the other side. Create a line of vertical holes on the metal drum starting from the top. This results in a large pool of gasoline. Shoot the lantern that hangs from the ceiling to blow it up.
When playing this level, you will encounter some zombies. To avoid them, duck. They will keep trying to attack, but you will not get hit. Note: This does not work with zombies with weapons.
When you get trapped in the room and the green gas floats in, use your glove to tip the statue back. When in that room, you will find the cat. You can drive it around.
When walking into the room where Cortez or Tipper says "Did I miss the party?", there is a slot machine. Walk over to it and press X to play slots. Press the buttons under "Hold" to hold that slot.
In the area just after the electric zombies, you will see a few rooms with a glass cage by them. On the nearby computer, you can have the subject inside the cage undergo electricity, incineration, shrinking, zombifying, stretching, cranium-expanding, and thinning. If you power up the stretching or cranium-expanding too much, the subject will explode. There are two computers in single player and one computer in co-op mode.
At the center room of the Zeppelin multi-player map is a hatch that opens. It will be very easy to figure out with the power-ups on. The platform that holds the Max Damage power-up is the detachable platform. Fire at the platform and the hatch below it will open. This causes the platform to fall out of the zeppelin, to the unlucky player(s)' doom.
To defeat The Princess, side-step in one direction while aiming up and turning. The closer that Cortez is to the fountain, the greater the chance to avoid being swiped. When the Boss is about to die, step back and fire the last shots to avoid the final death blow.
Use the Ghost Gun and position yourself carefully. Find a rock that shields Cortez's legs while allowing him to fire back. The fire attack from the Boss must drop to the floor before moving forward. The rock will protect you from most of the fire damage. Use a melee attack to kill the bats and allow the Ghost Gun to damage the tank.
In the Brotherhood house, you will be trapped in a large room with zombies. Take out the zombies with your pistol and baseball bat, but save all the shotgun shells that you can. When you defeat them all, the Deer Haunter crashes through the wall. The key to defeating it is to walk in circles around the beast in the middle of the room, continually shooting it in the torso with your shotgun while taking minimal damage. When it is defeated, the door to the next room will open. Additionally, in the room where the Deer Haunter is found, use your baseball bat to kill all the zombies. Do not use your guns; save them. When the Deer Haunter comes out of the wall, run past him and into his cave. Put your back to the wall and shoot him. He will not be able to hit you.
When the Deer Haunter breaks out of the wall, quickly run into the small opening where he just broke out of and start shooting. The Deer Haunter will be directly in front of you swinging, but he cannot hit you while your in there.
When you fight the giant eye monster (Princess), run right up against the well and shoot. You should not be able to get hit. Additionally, Princess can easily be defeated with the Harpoon weapon. Just wait for the beast to open its mouth and fire away, aiming for the metal container in said mouth. Three harpoon shots on target and Princess will belong to the past.
These are the huge robots on the Machine Wars and Something to Crow About levels. When you see these robots, charge them until you run into their chest. When you are this close they cannot shoot you because their arms are too long. Then, shoot them in the head or, if you want to save ammo, punch them to death.
In Crow's Palace in the Something To Crow About level, after crossing the bridge you activate using the electrotool, open the big doors. An intermission sequence begins. Wait for the unmoving (but laughing) Crow to move. As soon as he takes his first step in the intermission sequence, press X to jump into the action. As soon as you press X, fire at Splitter Crow for a few seconds. He will die without fighting. This is because when you skipped the intermission sequence, Crow had no health; by shooting him when he has no health he dies immediately.
Go to any ramp and stay out of the tank's sight. Use the Telekinetic Glove to take the ammunition when it spawns and attack the tank with rockets.
Before you enter the room with the beast (at the checkpoint), take out all of the enemies in the room, then on the ground floor. Pick up the health and armor at either ends of the room. Go upstairs and turn right. Follow the landing around to the end to pick up some very powerful plasma grenades. Then, go into the room at the top of the stairs to face the tank creature. This beast spawns exploding worms from its rear. Remain close to the pillars to avoid being incinerated by the spreading explosions. As for the beast, aim your explosives and guns at the weapons it is packing on his back. These include lasers, machine guns, rockets and others. At any opportunity you get, throw the five plasma grenades you have and try to get them to stick to the weapons that have not been destroyed yet. These do a lot of damage to the beast. Keep hugging the pillars for cover. After awhile, you will have most of his guns. Do not aim for the organic parts of the creature, as it takes no damage. When it is defeated, you will meet Jacob Crow again.
In the first mission, when you man the machine gun to defend the base you will hear someone screaming "They're coming out of the walls! They're coming out of the goddamn walls!" This is a reference to the movie Aliens.
On the level were you kill "Princess", the contraption that Jo-Beth is on is the fire cage from Indiana Jones And The Temple Of Doom.
When you are at Crow's computer, when you activate the countdown/self-destruct sequence, the monitor will display the message "You didn't say the magic word" and will have a little guy laughing and shaking his finger.
Story level "Breaking And Entering" is a mission that acts much similar to the Perfect Dark level "DataDyne Central: Defection". Both involve being on a rooftop, taking out security, and making your way to someone's private office, and then into the labs below the building. Also, like Joanna Dark, Amy Chen is also investigating a company's research. In "Something to Crow About", R-110 unexpectedly gets its own personality, much like Dr. Caroll.
When you are trying to help Past Cortez from being killed by the TimeSplitters on the beach, you can hear one of the men yelling "Come on! Do you want to live forever!".
Story levels "Machine Wars" and "Something to Crow About" play similar to "Robot Factory" in TimeSplitters 2, considering that you fight robots similar in ability to ChassisBots and SentryBots. Also, you get involved in taking out energy sources, much like the energy nodes in "Robot Factory".