Like any game using Valve's Source engine, there are a lot of internal settings users can change through the developer console. Alien Swarm is no different with plenty of commands to change how you play the game and its appearance.
If you don't know how to enable the console, go to Options, then Keyboard/Mouse, and click the checkbox that says "Enable Developer Console". Now you can open up the console mid-game or in menus by pressing the "~" key.
Want to play the game in a first-person view? Type firstperson, asw_hide_marine 1, and asw_controls 0 while you have the console open (try experimenting with them one at a time to see their effect). This is the result:
It's like Left 4 Dead with aliens.
There are a lot more settings to be found in the source so change as much as you like to your tastes. If you're new to console commands, just know that the "Server Side" commands provided are considered cheats so only those playing offline or server admins can change them.