Main •
Jana Carpenter as Kennedy Fisher, Heawood's long-time
Mystery Machine co-host who specializes in on-site investigating • Carpenter also portrays Daisy Marsh, a woman who looks identical to Fisher, in season 3. •
Barnaby Kay as Matthew "Matt" Heawood, the co-host of the
Mystery Machine who specializes in research and post-production •
Nicola Walker as Dr. Eleanor Peck (recurring season 1–2; main season 3–4), a university lecturer who specializes in myths and the occult, and becomes a regular advisor for the podcast. It is implied that she is in fact Marie Lairre, a character portrayed by Walker in Simpson's previous series
Mythos, having forgotten her past to go
undercover as Peck; she seemingly regains her memories in the season 4 finale.
Recurring •
Samuel Barnett as Charles Dexter Ward (seasons 1, 3), a patient in a psychiatric facility whose inexplicable disappearance is investigated by Heawood and Fisher • Mark Bazeley as: • Dr. Jonathan Willett (seasons 1–3), Ward's former psychiatrist who was later placed in a psychiatric institution after brutally murdering a seemingly random woman • Albert Wilmarth (seasons 2–3), a vicar and friend of Akeley named after Albert N. Wilmarth, the narrator of Lovecraft's
The Whisperer in Darkness. In season 3, he is revealed to be "John Silence", a man working for the Department of Works, a mysterious organization with ties to the supernatural. •
David Calder as Henry Akeley (season 2), a former student of Peck who goes missing, leading Fisher and Heawood to go look for him and look into his life •
Karla Crome as Melody Cartwright (season 3), a young woman who becomes the target of an elusive group in season 1, and later meets Heawood • Samantha Dakin (season 1) and
Jennifer Armour (season 3) as Alice, a young county clerk from Providence whom Fisher meets in the first season, and again in season 3, having become a fan of the show •
Phoebe Fox as: • Lucy Hawthorne (season 1) • "Parker" (seasons 2–3), a member of the Department of Works. The character returns from Simpson's previous series
Mythos, in which she was already voiced by Fox. •
Adam Godley as George Shepley / "Dr. Allan" (season 1), a librarian who becomes close to Fisher and helps her in her investigation •
Rebekah Staton as April Marston (season 2) • Walles Hamonde as Zadok Allen (season 3), a historian Fisher meets in Innsmouth, and who appears identical to George Shepley •
Kate Isitt as Caroline Morse (season 4), the estranged sister of Wilberforce Ashton-Heath •
Susan Jameson as Amelia Fenner (season 1), a British practising witch involved with a number of covens in the mid to late 20th century. •
Ferdinand Kingsley as "Slide" (seasons 2–4), a hacker and friend of Fisher who sometimes assists in her investigations •
Steven Mackintosh as Jasper (season 3), an old friend of Heawood who works for the government •
Kyle Soller as Casey (season 3), a man who runs a hotel in Innsmouth •
Rufus Wright as Wilberforce Ashton-Heath (season 4), an
alt-right Member of Parliament who is pro-
Brexit and anti-immigration. • Ben Crowe as Marcus Byron (season 4), a former member of the Department of Works. • Catherine Kanter as Victoria Ness (season 4), a former member of the Department of Works and Byron's wife. ==Series==