It was originally intended that Chris O'Dowd's cop would be American, but everyone was so enamored of O'Dowd's native Irish accent that it was decided that he keep it.
The cast spent about two weeks improvising with each other, a lot of which was incorporated into the movie.
Before he came to Hollywood, Jon Hamm, who plays Ted in this movie, was a high-school drama teacher at the John Burroughs School in Ladue, Missouri. One of his acting students was Ellie Kemper, who plays Becca.
In the original script, the bachelorette party made it to Las Vegas, Paul Feig had even prepped the scenes they planned to shoot there. However, after the release of The Hangover (2009), Feig felt that they did Las Vegas so well, anything Bridesmaids did would pale in comparison and decided to change the sequence. When discussing funny alternatives to a wild girls weekend in Las Vegas, someone suggested "well, what if they never get there?" This led to the addition of the scene on the airplane.