I actually found this by mistake while getting the trading cards! I was just letting Postal idle on the title screen and when I returned I found I had not only the cards but some achievements as well!

How to do: Launch the game and get to the screen where you have Start, settings and quit as your three choices. Stop there, turn off your speakers and screens to save them, and go to bed! What happens is there's a demo of the game that launches every x minuntes of that screen being up! And guess what! It's progress counts for the following achievements:

Forrest Gump
Patrick Bateman
Dexter
Holes is Holes
Mickey and Mallory
Bulletstorm
Peter North

By NO means is this the fastest way to theses grindy achevements, but it is the easiest with ZERO effort being put forth by the gamer! Use it between sessions to boost up the progress of these massive undertakings or to get a head start on them. Or do this every night for about 20 days or so / every time your not on your PC and just let it do them for you! Your chioce! Happy hunting!