What are the cheats for undead nightmare?

Answer 1

You can enter the following codes in the Cheats Menu under "Options." Enter the entire phrase given below to activate the cheat, punctuation included. Capitalization does not matter. Once a cheat is entered you may toggle them from a master list. If the cheat effect isn't made obvious by its title, we give a description of it in parenthesis below. Entering cheats disables saving and Trophies / Achievements! Finally, cheats can often be found in a box at the bottom of the newspapers you can read in towns.

• Invincibility - HE GIVES STRENGTH TO THE WEAK
• Infinite Dead Eye - I DON'T UNDERSTAND IMNFINITY
• Infinite Horse Stamina - MAKE HAY WHILE THE SUN SHINES
• Infinite Ammo - ABUNDANCE IS EVERYWHERE
• Money ($500) - THE ROOT OF ALL EVIL, WE THANK YOU!
• Horse - BEASTS AND MAN TOGETHER
• Diplomatic Immunity (Marshalls ignore crime) - I WISH I WORKED FOR UNCLE SAM
• Decrease Bounty - THEY SELL SOULS CHEAP HERE
• Gun Set 1 - IT'S MY CONSTITUTIONAL RIGHT
• Gun Set 2 (buffalo rifle, sawed-off shotgun, semi-auto pistol, Schofield revolver, Winchester repeater, and fire bottle x3) - I'M AN AMERICAN. I NEED GUNS
• Who? (Turn into a Nobody) - HUMILITY BEFORE THE LORD
• Old School (Sepia Filter) - THE OLD WAYS IS THE BEST WAYS
• Sharp Dressed Man (Unlock Gentleman's Suit) - DON'T YOU LOOK FINE AND DANDY
• Lewis and Clark (Unlock all areas) - YOU GOT YOURSELF A FINE PAIR OF EYES
• Jack Attack (Change John into Jack) - OH MY SON, MY BLESSED SON
• Man In Uniform (Unlock US Marshal, US Army and Bureau Uniforms) - I LOVE A MAN IN UNIFORM
• Good Guy (Get a good reputation) - IT AINT PRIDE. IT'S HONOR
• Gang Chic (Unlocks Treasure Hunter Ouftift) - YOU THINK YOU TOUGH, MISTER?
• Hic (Permanent "drunk" effect) - I'M DRUNK AS A SKUNK AND TWICE AS SMELLY
• Coach (Spawns a coach) - NOW WHO PUT THAT THERE?
• Famous (become instantly Famous) - I AM ONE OF THEM FAMOUS FELLAS

Answer 2

Find them on the internet and enter them by going to the options menu, clicking cheats and enter the cheats codes, you can also toggle your cheats from here.