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 second bedroom scene between Annie and Ted, Annie tells Ted that she will take the fictional "George Glass" to Lillian's wedding instead of him. "George Glass" was also the name of Jan Brady's fictional boyfriend on The Brady Bunch (1969).
Rose Byrne was better known for being a dramatic actress, in projects like Sunshine (2007) and Damages (2007), when she was cast as Helen. This was mainly down to her performance in Get Him to the Greek (2010). However, everyone on Bridesmaids (2011) was genuinely surprised at Byrne's ability to do improv comedy during the shoot, best exemplified in the scene at the engagement party where she tries to usurp Annie's tribute speech.
Judd Apatow pushed hard for wild, physical comedy, while Annie Mumolo and Kristen Wiig preferred to go for subtler comedy. The film's most notorious sequence--the diarrhea scene--was largely at the insistence of Apatow and director Paul Feig.
The project was written over a period of several years. Kristen Wiig had her day job on Saturday Night Live (1975) in New York City while Annie Mumolo would work on the script in Los Angeles. The two would meet at weekends and compare notes and conduct table reads, often with Judd Apatow in attendance to get his notes.
The trivia items below may give away important plot points.
In the DVD commentary, it is mentioned that director Paul Feig and Kristen Wiig made a deliberate decision for Annie to keep her bra on during her sex scenes with Ted (Jon Hamm), but to have her take it off during her sex scene with Nathan (Chris O'Dowd), as a way of symbolizing that Annie feels she can open up emotionally (as well as physically) to Nathan in a way that she cannot to Ted.