Fake Nationality: American Robert Downey, Jr. played an Australian who was playing an African American and even disguised himself as an Asian farmer. He's a dude playing a dude playing as a dude disguised as another dude.
The Vietnamese cartel Chinese Dragon is lead by a Chinese-American and the second-in-command is Filipino-American.
The Other Marty: Owen Wilson was originally set to play Tugg Speedman's agent, Rick Peck, but after his suicide attempt, he understanbly backed out of the role. The role ended up going to Matthew Mc Conaughey.
Lampshaded and mocked for the MTV Movie Awards intro of 2008: Ben Stiller tries to ramp up popularity by billing Jack Black as Po and RD Jr. as Iron Man on the poster. From there, he creates a viral video with the twosome, the first take of which has them in masks of their respective hit movie characters. Iron Man kicks Po in the crotch, and it just gets worse from there for Black. It ends with him getting his face exploded. Stiller tries to pass it off as realistic special effects to an understandably annoyed RD Jr.