Everyone on the Disc has heard of Cohen the Barbarian, but what about the other heroes (and not-quite-heroes, and anti-heroes) who are masters of not dying? Sure, there's the usual Guild meetings (for those who are Guild members) where the youngsters pat you on the head, tell you that you're a legend, and then leave you behind while they go off and have all the fun. There's only so many Early Bird specials that one can enjoy at Mrs. Kurfle's diner before you just have to go off on One More Adventure. (Unless it's meatloaf day. Mrs. Kurfle makes the best meatloaf ever.)
Players are residents of the Disc who had lives filled with adventure (on purpose or not) but are now in their "golden years" and itching for just one more go. There are only two restrictions: 1) you can't be any canon character (you can be BASED on one, as long as you have a different name), and 2) you have to be in Ankh-Morpork when the game starts. (How else would you get to have Mrs. Kurfle's meatloaf? All attempts to ship it elsewhere have resulted in a greasy mess and a trail of vermin.)
Players do not have to be familiar with The Expendables at all. A little bit of familiarity with Terry Pratchett's Discworld will help, but you need not be steeped in the knowledge of All Things Disc(tm)-- you just need enough to know the tone of the series and what sort of people might be roaming around bored from failing to die.