The show was written specifically for Sarah Lancashire, who had previously had a starring role in Sally Wainwright's Last Tango in Halifax (2012). Wainwright was so impressed by Lancashire's performance, she devised Happy Valley (2014) as a solo vehicle for Lancashire.
The show is filmed around Halifax in Yorkshire, and nearly all of the actors are local to the area. They speak with the distinct Yorkshire accent and use phrases like "it were" (instead of "it was") and "it's reet" (meaning "it's alright").
Following the second series in 2016, it was announced that Sarah Lancashire was unlikely to return to her lead role as Catherine Cawood; though it was announced several days later that Lancashire had had second thoughts and would be up for returning.
Although James Norton grew up in Yorkshire, he attended a private boarding school and Cambridge University, and normally speaks with a very "posh" accent. He first auditioned for the role of Tommy Lee Royce by submitting a video, as he was out of the country at the time. He later said he thought auditioning this way helped him get the role, as actors with posh accents are often typecast by producers who don't feel they are authentic to play criminals and thugs. He has appeared on several talk shows where the hosts were shocked to hear him speak, as the Yorkshire accent he has in "Happy Valley" is so authentic. He uses his normal accent in Grantchester (2014) that takes place near Cambridge.