Quick Time Unlocks :
You can get extra cheats and characters if you beat the following times on levels:
[Submitted by: Markus]

Tomb -
Easy-1min (paintball cheat)
Normal-2min35secs (priestess character)
Hard-2min20secs (teeth mummy character)

Chinese -
Easy-1min (chinese waiter character)
Normal-2min35secs (gun sounds cheat)
Hard-6min5secs (mr. big character)

Extra arcade mode features:
Successfully complete the first three levels in story mode to unlock the Village, Chemical Plant, and Planet-X levels in arcade mode. Additionally, Dr. Seth Graven, Dr. Katje Nadir, Fingers McKenzie, Ravelle Velvet, Sebastian Photon, and Angel Perez Bots will also be unlocked as playable characters in arcade mode.

Cultist :
Successfully complete the 1935 Tomb level in story mode under the easy difficulty setting to unlock Cultist in arcade mode.

Chinese Chef :
Successfully complete the 1970 Chinese level in story mode under the easy difficulty setting to unlock Chinese Chef in arcade mode.

Badass Cyborg :
Successfully complete the 2005 Cyberden level in story mode under the easy difficulty setting to unlock Badass Cyborg in arcade mode.