Lewis MacDougall

Conor

Sigourney Weaver

Grandma

Felicity Jones

Mum

Toby Kebbell

Dad

Ben Moor

Mr. Clark

James Melville

Harry

Dominic Boyle

Anton

Jennifer Lim

Miss Kwan

Max Gabbay

Steven

Morgan Symes

Lawyer

Frida Palsson

Lily's Mum

Wanda Opalinska

Female Nurse

Lily-Rose Aslandogdu

Lily

Liam Neeson

The Monster (voice)

Geraldine Chaplin

The Head Teacher (uncredited)

Jake Talbot

Rollercoaster Boy (uncredited)