The Mark
In The Mark, try shooting your English co-combatant in the head. He's so blinded by love for your rugged American good looks that he tolerates it without blinking. Stab him in the calves, however, and he complains with a grumpy: "Oh, I say." This is probably how English squadrons react to friendly fire all the time.
Prey
When you're eventually forced to kill your girlfriend, test her love for you by killing her very slowly with the wrench. In the Cherokee afterlife, will she bear a grudge, saying, "Thanks for saving the world, Tommy, not to mention smacking up my chops with a rusty monkey wrench?" Er, no.
The Jackass conclusion
No one likes being told what to do. If you were going down some stairs and there was a barrier blocking the way just before the floor you wanted, you wouldn't go back up three flights and solve a riddle to unlock another staircase. You'd kick over the barrier and do a little dance to show how you've beaten the system.
And that's how it should be with games. Fight against being pushed down every corridor. Shoot every TV, crowbar every corpse and crouch to groin level whenever an NPC gives you any exposition on the plot. The developers have to trust you with some free will, otherwise you're just watching a movie. Put a lot of effort into abusing that trust, and you can have as much fun as playing the game "properly."
