Lewis MacDougall as Conor

Sigourney Weaver as Grandma

Felicity Jones as Mum

Toby Kebbell as Dad

Ben Moor as Mr. Clark

James Melville as Harry

Oliver Steer as Sully

Dominic Boyle as Anton

Jennifer Lim as Miss Kwan

Max Gabbay as Steven

Joe Curtis as Peter

Morgan Symes as Lawyer

Max Golds as 5-Year-Old Connor

Frida Palsson as Lily's Mum

Wanda Opalinska as Female Nurse

Patrick Taggart as Teacher

Lily-Rose Aslandogdu as Lily

Liam Neeson as The Monster (voice)

Geraldine Chaplin as The Head Teacher (uncredited)

Jake Talbot as Rollercoaster Boy (uncredited)

Lee Bolton as Rollercoster Rider (uncredited)

Garry Marriott as Paramedic (uncredited)