The show's remaining three regulars return to the series. John Barrowman returns as Captain Jack Harkness, an immortal ex-con man from the 51st century who first appeared in the 2005 Doctor Who episode "The Empty Child". In the first three series of Torchwood, Jack ran the Cardiff remnant of the British organisation the Torchwood Institute, and was "the happiest he'd ever been". After the destruction of Torchwood and the deaths of his grandson, friends, and his lover Ianto Jones in the third series, the only thing that could bring Jack back to Earth is "his unstated love for Gwen Cooper". Gwen, portrayed by Eve Myles, became pregnant in the third series by her husband Rhys Williams (Kai Owen), the third returning star. Since the fall of Torchwood, they have begun a quiet life living in seclusion with their baby, Anwen. Secretly, she is bored since the fall of Torchwood and pines for her former, extraordinary life; though she loves her husband and child, she retains feelings for Harkness. Other returning characters include Gwen's former police partner PC Andy (Tom Price), now a sergeant,and parents, Geraint (William Thomas) and Mary Cooper (Sharon Morgan), who previously appeared in the 2008 Torchwood episode "Something Borrowed". Morgan will appear in six of the ten episodes, whilst Thomas' online CV intimates that he will be a 'regular' character.
Mekhi Phifer joined the show as Rex Matheson, the CIA's "golden boy" operative, fast-tracked for career stardom. Highly intelligent and heroic, he teams up with Jack and Gwen to solve the mystery behind "Miracle Day".He is joined by Esther Drummond (Alexa Havins), a CIA watch analyst with an optimistic faith in humanity. Arlene Tur has been cast as surgeon Vera Juarez, whilst Bill Pullman plays Oswald Danes, a dangerous convicted paedophile and murderer who turns a prison release into celebrity status. Oswald faces lethal injection, but becomes swept up in the story's plot thread when he — like the rest of the world — simply does not die.Lauren Ambrose has been cast as Jilly Kitzinger in seven of the ten episodes. She will play "a sweet-talking PR genius with a heart of stone who's just cornered the most important client of her career … and maybe of all time". Lena Kaur joins the UK cast as Dr. Alicia Patel. Dichen Lachman, Wayne Knight, Ernie Hudson, C. Thomas Howell, and John de Lancie, all have guest starring roles. Dillon Casey will play Brad, a one night stand of Captain Jack's.
Casting calls differed from the final press release. Pullman and Havins' characters' surnames were originally Jones and Katusi, respectively. Additionally, calls originally specified that Matheson was to be played by a white actor and Katusi by a non-white actress. One Tree Hill's Chad Michael Murray and Dollhouse's Enver Gjokaj both auditioned for the part of Matheson. Greek star Amber Stevens also auditioned for the part of Katusi/Drummond. Actor James Marsters expressed strong interest in returning for the fourth series as his series two character Captain John Hart.