Speaking in 2020, Raiff described the plot as following two siblings and their single dad who is making them grow up too fast. With that in mind, the 7-year old and 9-year-old characters would be cast with adult actors.[3]
Speaking in 2020 to Screen Daily, Cooper Raiff said he had been writing the script for Hal & Harper for "a long time".[4] In August 2023, Mark Ruffalo and Lili Reinhart were cast, with Ruffalo expected to play the father and Reinhart and Raiff expected to play the siblings. Also joining the cast were Addison Timlin and Havana Rose Liu.[5] The series is produced by Bad Bangs LLC and obtained an interim agreement to continue production during the 2023 SAG-AFTRA strike. Raiff is also producing through Small Ideas alongside Clementine Quittner.[6]
Filming started in Los Angeles in August 2023.[7] In January 2024, lead actress Lili Reinhart shared photographs from filming on her private Instagram account.[8]
