The pilot episode featured a different cast than the series: Andrea Riseborough played Annie and Guy Flanagan played Mitchell, but both were recast with Lenora Crichlow and Aidan Turner when the show went into full production. Russell Tovey was the only main cast member to remain from the pilot.

Creator Toby Whithouse was originally approached by Touchpaper Television to develop a drama about friends buying a house together, an idea he wasn't enthusiastic about; after months of struggling to find a storyline, he eventually invented the supernatural elements that became the show's premise.

The pilot was commissioned in 2007 by BBC Three controller Danny Cohen as one of several pilots meant to rebrand the channel, and Whithouse was initially told only another pilot, Phoo Action, would be picked up for a full series.

After the pilot aired in February 2008, journalist Narin Bahar of the Reading Chronicle started an online petition signed by over 3,000 people lobbying BBC Three to commission a full series, which helped turn the pilot into an ongoing show once Phoo Action was scrapped for having weak scripts.

Russell Tovey announced he was leaving the series after the first episode of Series 4 to work full-time on his other show Him & Her, while Sinead Keenan revealed she had not filmed any Series 4 scenes and would exit off-screen.

The first two series were filmed in Bristol, using locations like Windsor Terrace in Totterdown for the trio's home and Bristol General Hospital and Glenside for the hospital scenes, while Series 3 onward moved production to Barry Island, Wales, partly due to the BBC's 'Out of London' initiative to relocate productions.

For later series filmed in Barry Island, the production used local resident Gary Rowe's house as the characters' bed-and-breakfast base, an abandoned bus depot converted into a film studio for interiors, and drama students from Coleg Gwent as extras.

The Series 3 premiere drew 1.8 million viewers across live, delayed and online platforms, the largest series-premiere audience in BBC Three history at the time.