Shared with you
The original ending featuring Gary and Brooke running into each other at an art fair each dating someone that looked identical to the other (Brooke's date looked like Gary, and Gary's like Brooke) was scrapped when it tested poorly. A new ending was filmed with the same result but featured Gary and Brooke running into each other on the street alone. In both versions the two remained broken up.
Vince Vaughn's parents have bit roles in the movie. His mother plays a tourist and his father plays Jennifer Aniston's father.
Vince Vaughn gained 26 pounds for the role. Vaughn was reported to have gained weight as a result of his recent decision to quit smoking.
In the opening scene at Wrigley Field, Jon Favreau makes a reference to going to "Wiener Circle" to have hot dogs after the game. Wiener Circle is a hot dog stand famous for the staff yelling and cursing at customers when they order. If the customer does not return the favor, they do not take your order. Generally, though, this only happens late at night on the weekends.
Before Brooke takes her "nude walk", she looks at herself in the mirror and says, "Who loves ya, baby". This was the catchphrase of Telly Savalas, Jennifer Aniston's godfather.
On the screenplay, Jennifer Aniston said that when she received it "It was clearly written by men". During two weeks of intense rehearsing, she and Vince Vaughn improvised while the writers took notes and, thanks to those sessions, the "female perspective" was brought to the story.