This sounds insane, but it comes directly from director Todd Phillips so I assume it's true.
When casting for the original "Hangover" was taking place, Jake Gyllenhaal was considered for the part of Alan. That's the part that Zach Galifianakis eventually got.
Obviously, the character wasn't going to be quite as bizarre or outrageous as he turned out to be. Phillips says, quote, "We were writing the brother-in-law as a younger brother they had to take along with them..
"Then we thought it'd be so much more awkward if it was an older brother who's still at home." They also considered Jonah Hill and Thomas Haden Church for the part.
Lindsay Lohan was also considered for the part of the stripper, Jade. But Phillips ultimately decided she was too young, and went with Heather Graham.
"The Hangover Part 3" comes out on Memorial Day weekend btw.. check out the trailer: