This is nuts and I know it, but here we go anyway.
I want some forum software to not run a forum at all. I want to be able to run it strictly locally, not on the net at all, and enter messages from different pseudo-users along with avatars, etc. so that it looks completely real. This is for a graphics application at work, and I know I could fake it in Photoshop, but since it's gonna be a lot of pages, it would be much easier to just have an actual software program that I could just enter the data into.
I've checked around and there are scores of different offerings and there really isn't any way that I've found to determine whether it does what I want it to do without downloading, installing, etc.
Platform? My suggestion is going to be just set it up to run as if on a server but just don't allow external access to the machine (assuming you are behind a NAT device). There are pre-built, ready to deploy (or customize) examples out there, but some have software (like PHP capability and a database server installed) and/or platform requirements (Windows vs. Linux/BSD).
Look into phpBB for a good, standard forum system. Here is a good example of how to get it running in Windows (which I assume you are using, if not there are plenty of howto's out there for other platforms)