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.
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.
Sally Wainwright revealed that one of the reasons she had chosen Hebden Bridge as the setting for the show was to highlight the drug problems the town has faced. This is mentioned in series one, with Catherine (Sarah Lancashire) repeatedly demanding for more to be done to combat drug dealing in the town.
George Costigan (Nevison Gallagher) & Siobhan Finneran (Clare Cartwright) also worked together on Unforgiven (2009) as Eddie Ackroyd & Izzie Ingram respectively. They'd previously worked together on Rita, Sue and Bob too (1987) as two of the title character, Bob and Rita.
The trivia item below may give away important plot points.
In Series 2, Kevin Doyle and Amelia Bullmore play two lovers, with Doyle ultimately murdering Bullmore's character after she begins blackmailing him. The pair had previously starred together in series 1 of Scott & Bailey (2011).