The Expendables (2010) establishes the team led by Barney Ross (Sylvester Stallone), which includes Lee Christmas (Jason Statham), Gunner Jensen (Dolph Lundgren), Hale Caesar (Terry Crews), and Toll Road (Randy Couture). The movie introduces their mission to overthrow a South American dictator and their personal dynamics.
The Expendables 2 (2012) builds upon the events of the first movie. It expands on the team's backstory and introduces new characters, including Trench Mauser (Arnold Schwarzenegger) and Billy the Kid (Liam Hemsworth). The sequel delves deeper into the characters' relationships and their motivations.
Knowing the character development and events from The Expendables enhances the experience of watching The Expendables 2. However, it is not impossible to enjoy the second movie without having seen the first one. The Expendables 2 provides enough context to understand the characters and the main plot.