Lewis MacDougall
Conor
Liam Neeson
The Monster (voice)
Sigourney Weaver
Grandma
Felicity Jones
Mum
Toby Kebbell
Dad
Ben Moor
Mr. Clark
James Melville
Harry
Oliver Steer
Sully
Dominic Boyle
Anton
Jennifer Lim
Miss Kwan
Max Gabbay
Steven
Joe Curtis
Peter
Morgan Symes
Lawyer
Max Golds
5-Year-Old Connor
Frida Palsson
Lily's Mum
Wanda Opalinska
Female Nurse
Patrick Taggart
Teacher
Lily-Rose Aslandogdu
Lily
Geraldine Chaplin
The Head Teacher (uncredited)
Jake Talbot
Rollercoaster Boy (uncredited)
Lee Bolton
Rollercoster Rider (uncredited)
Garry Marriott
Paramedic (uncredited)