Jean-Claude Van Damme
was personally offered a role by Stallone, but turned it down.
This reportedly led to Dolph Lundgren
being cast instead. The role accepted by Forest Whitaker
was originally conceived as a role for Wesley Snipes
but later rewritten for Whitaker.
Due to a scheduling conflict prior to filming, Forest Whitaker was replaced by Curtis Jackson (aka 50 Cent)
for the role of Hale Caesar,
who was then in turn replaced by former NFL player Terry Crews
Stallone wanted Sandra Bullock to play the role of Agent Diane Lickson yet Bullock's schedule did not work with the film, and she could not participate. 
Ben Kingsley was offered the role of Monroe before Eric Roberts was cast. The role may be combined with that of Agent Will Sands. 
The role of the man who hires the Expendables, Mr. Church, has been very difficult to cast. Initially offered to Schwarzenegger, he declined in favor of a smaller cameo in respects to his political career.
The role was then offered to Kurt Russell whose agent replied that he was not interested in "Ensemble acting at the moment." 
Currently, Stallone is looking to find a big action name to cameo as Mr. Church with rumors suggesting that the role has been offered to friend and fellow former Planet Hollywood co-owner Bruce Willis
, although he is apparently tied to the filming of A Couple of Dicks
. On May 29, 2009, the script under went a few re-writes which ultimately excluded Brittany Murphy
's character. Steven Seagal
was also asked to play a cameo role, but turned down the offer due to negative connections with the producer Avi Lerner