Play as Alien:
Another way to be the alien besides how that one guy said is this, "To be an
alien shoot the first three humans you see and nothing else. After you shoot the
first human s.t.a.a.r. team troops you will not be harmed by any gun fire so dont worry.
After you
shoot the third guy, you will become the alien in the main hallway for the first
level.

Start with a Shotgun:
At the pause screen press (with a controller not a gun) triangle, square,
triangle, left, R1 to start with a shotgun.

Best and Worst Endings:
For the best ending, shoot off all the panels on the Mothership. For the worst
ending, let the Mothership escape without hitting any panels.

Secret Rooms:
It seems only proper that a secret base contains secret rooms. Here's how to
find them:

Kronn Hunter Mode:
Shoot the first three S.T.A.A.R. team members at the beginning of the game. If done correctly, you
will enter Rebel Kronn Hunter mode.

Level 2: Shoot out all of the upper windows of the far back wall in
    the Hangar. Another can be found by shooting all the yellow barrels
when you are the forklift.

Level 4: Shoot every yellow box while you're riding on the STAAR jeep

Level 5: Shoot the doorplate of the second door marked General
    Weatherby as the door is opening.

Level 7: Shoot all the yellow barrels at the first area where the four
    purple zombies are throwing barrels on the hill.