When trying to secure Philip Seymour Hoffman for his cameo his agent claimed he was too busy so Ricky Gervais requested his email address and sent him the following: "Dear Philip, will you please appear in my new film? There is very little money involved as I spent the budget on testicular implants, but don't look upon them as my testicles, look at them as our testicles." Phillip Seymour Hoffman couldn't refuse after that.
Ricky Gervais insisted that Louis C.K. shave off his goatee because he needed to look like a caveman for the opening scene. C.K. was very reluctant but he eventually shaved it off. The cavemen scene was filmed but it was deleted from the main feature, and presented as a short film on the DVD.
Director, writer and actor Ricky Gervais sponsored an online Photoshop contest related to the movie theme by asking contestants to edit known movie posters as if those posters were usually lying to the audience.
The film was released theatrically in France with its original title, but by the time it was released on DVD, this was changed to "Mytho-Man!", which is a play on words, the French word "mythomane" meaning "pathological liar".
The trivia item below may give away important plot points.
The Chapel Anna (Jennifer Garner) is set to marry Brad (Rob Lowe) in, is the "Martha-May Chapel at the Wayside Inn" (located in Sudbury, Massachusetts). Funnily enough, not only is this the same Chapel Jenny (also played by Garner) gets married during Connor's (Matthew McConaughey) 'future' fantasy in Ghosts of Girlfriends Past (2009); but her groom's name is also Brad (played by Daniel Sunjata). Additionally, neither wedding eventuates due to the main character in both films winning her heart instead; and, both films were released the same year (2009).