Stefanie Scott plays Quinn Brenner, the teenager who visits Elise Rainier hoping to reach her mother, then breaks both legs in an accident and is confined at home with a presence she cannot escape. Deadline reported in June 2014 that she had joined the cast, presenting her as the star of A.N.T. Farm and Jem and the Holograms, and TheWrap later described her as the young girl whom Elise helps.
Lin Shaye plays Elise Rainier, the retired medium who senses danger around Quinn and warns her not to contact the dead again. Shaye played Elise in Insidious (2011) and Insidious: Chapter 2 (2013), and TheWrap reported in October 2014 that she was reprising the role for this prequel, which follows the psychic before the events of the first film.
Dermot Mulroney as Sean Brenner
Dermot Mulroney plays Sean Brenner, Quinn's father, who is raising her and her brother Alex after their mother died a year earlier. Leigh Whannell announced the casting on Twitter on June 25, 2014, writing that Mulroney had been cast as one of the leads, according to TheWrap, which also pointed to his recent work in the NBC drama Crisis and in August: Osage County.
Leigh Whannell plays Specs, one half of a small ghost hunting team, a role he also played in the first two films. He wrote and directed Insidious: Chapter 3, his feature directorial debut. TheWrap reported his appointment as director on May 7, 2014, with James Wan, his Saw co-creator, producing alongside Jason Blum and Oren Peli.
Angus Sampson plays Tucker, Specs's partner in the ghost hunting business. TMDB credits him as Tucker in Insidious (2011) and in Insidious: Chapter 2 (2013), which makes this prequel his third outing in the role. As in both earlier films, his billing sits directly below Whannell's, so the pairing stays together in the credits as well as on screen.
Steve Coulter plays Carl, a character he first played in Insidious: Chapter 2 (2013), where he was also billed sixth, the same position he holds here. In that film TMDB also credits Hank Harris as Carl (adult), while in Chapter 3 Coulter is the only actor credited in the part, which makes him one of the clearest links between the two films.
Hayley Kiyoko as Maggie
Hayley Kiyoko plays Maggie and is billed seventh, the highest placed newcomer after the Brenner leads. She has no credit in the TMDB cast lists for Insidious (2011) or Insidious: Chapter 2 (2013), so this prequel is her first appearance in the franchise, which puts her among the new faces surrounding the returning Elise and the ghost hunters.
Tom Gallop as Dr. Henderson
Tom Gallop plays Dr. Henderson and is billed eighth. TMDB lists no role for him in Insidious (2011) or Insidious: Chapter 2 (2013), so Chapter 3 is his first appearance in the franchise. With Quinn injured and confined at home, the film draws on a small medical supporting cast, and Dr. Henderson is credited alongside a Nurse and a Male Nurse.
Tate Berney as Alex Brenner
Tate Berney plays Alex Brenner, Quinn's brother, who lives with her and their father, Sean, in the apartment where the haunting unfolds. He is new to the franchise, with no credit in the first two films on TMDB, and he completes the Brenner family on screen alongside Ele Keats, credited as Lillith Brenner, the mother whose death a year earlier sets Quinn's search in motion.
Corbett Tuck as Danielle
Corbett Tuck plays Danielle and is billed tenth. She returns to the franchise from the original film, where TMDB credits her as Nurse Adele / Doll Girl #2, so Chapter 3 brings her back in a different role rather than the same character, and her credit links this prequel directly to the first film in the series.
Tom Fitzpatrick as Bride in Black
Tom Fitzpatrick plays the Bride in Black and is billed eleventh. TMDB credits him in Insidious: Chapter 2 (2013) as Bride in Black / Parker Crane (old), so his casting carries the same figure from the earlier film into this prequel. He is one of the few supporting performers who returns to a character rather than taking on a new one.
Jeris Poindexter as Harry
Jeris Poindexter plays Harry and is billed twelfth. TMDB does not list him in the cast of Insidious (2011) or Insidious: Chapter 2 (2013), so Chapter 3 is his first appearance in the franchise. Harry is one of the smaller supporting parts among the 27 credited roles, with two more performers listed by TMDB as uncredited.