The film was originally conceived and written as a heterosexual love story.
The title comes from the first four words of the song "Happy Together" by The Turtles, also used as the end credits song.
The film takes its title from the song "Happy Together," by The Turtles. Writer and director Ol Parker reveals on the DVD audio commentary that the film was originally titled Click, after the French term for love at first sight, but conflicts with the Adam Sandler film Klick (2006) necessitated the name change.
In the video store, Celia Imrie notices a stand-up cut-out of her Star Wars: Episode I - Die dunkle Bedrohung (1999) co star Ewan McGregor (promotional still from Long Way Round - Ein wahres Abenteuer (2004)) and remarks on his attractiveness. McGregor's wife, Eve Mavrakis, was the production designer for Eine Hochzeit zu dritt (2005).
In the audio commentary the director mentions that the hat Rachel (Piper Perabo) is wearing during the soccer game is actually a crew member's. She was cold and tried on different people's hats until she found one she liked.