To play in first person you must be playing practice (single player) or multiplayer with sv_cheats set to 1 (disables achievements).

Console:

Go to your steam library, right click on Alien Swarm and open properties -> "Set launch options".
Type in "-console" (without quotes). If you now start the game you should see the console, I would advice to bind the console to a key, like F11. Type in the console "bind F11 toggleconsole"

Enter these commands into console:

"firstperson" (change view to first-person)
"asw_hide_marine 1" (makes you invisible to yourself so you can see properly)
"asw_controls 0" (Changes controls to more FPS style of controls)


FPS View

Notes:

• Can't hack in first person (asw_controls back to 1 to hack)
  
• Can't jump
  
• Grenades are broken
  
• "thirdperson" (Snaps view back to third-person)

Paste the following into your config.cfg (stored in Steam\steamapps\common\alien swarm\swarm\cfg) at the bottom, then your are able to use F11 to toggle first and third person views.
This way you can easily change the view between FPS and third-person view.

Code:

alias "FPSView" "fpsview1"
alias "fpsview1" "firstperson;asw_hide_marine 1;asw_controls 0; alias FPSView fpsview2"
alias "fpsview2" "thirdperson;asw_hide_marine 0;asw_controls 1; alias FPSView fpsview1"
bind "F11" "FPSView"

To unlock the promotion stars reach level 27 and hit promote. It will reset your levels, but you will have the promotion stars.

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