Press TIDLE to drop the console and enter the follwoing cheats:
Invincibility ... free lives
God mode on/off (enemy) ... aigodmode <0> or <1>
God mode on/off (protectee) ... aiprotectprotectee <0> or <1>
God mode on/off (self) ... god <0> or <1>
Enemy hearing on/off ... aidisableears <0> or <1>
Enemy sight on/off ... aidisableeyes <0> or <1>
Clipping on/off ... noclip <0> or <1>