The actor voices Hazel in the new adaptation of Richard Adams’ bestselling novel about a small group of rabbits searching for a new home, which features a starry cast including John Boyega, Nicholas Hoult, Rosamund Pike, Olivia Colman and Gemma Arterton.
Talking about making Hazel’s sounds, McAvoy asked co-star Arterton if she ever got “really light-headed”, and she agreed.
“I’ve almost passed out a couple of times,” the actor said.
Arterton, who provides the voice of Clover, said playing the animal was “liberating” and took her back to her childhood.
She said: “You have to do this pass, it’s like an ‘effort pass’, which is where you record, ‘oh, you’re running away from a cat now and you’re sniffing the air, and you’re running away from a seagull’, and you just have to be a child really.
“And it’s always fun, you always do it at the end because you might hurt your voice because you have to scream and things like that, but it’s actually quite liberating - you just have to let yourself be a complete idiot.”
Hoult, who plays Fiver in the BBC/Netflix co-production, said he enjoyed using just his voice.
“It’s actually lovely to not have any of the other kerfuffle of film-making going on around you and just focus very much on the voice and what you’re aiming to do,” he said.
The adaptation was directed by Noam Murro, who said Watership Down would “always be relevant”.
“Turn on any news channel and you’ll find the answer,” he said.
“The brilliance of Watership is that it uses a tale about migrant rabbits to hold a mirror up to the human experience, which is a cyclical journey that we can all relate to.”
He said the story touches on “timeless themes: the meaning of home, migration, who we are and how the stories we tell shape our society”.
Watership Down will air as two feature-length episodes on BBC One on Saturday December 22 and Sunday December 23.