In an email to the director James Marsh about the portrayal by Eddie Redmayne, Stephen Hawking said there were specific points when he thought he was watching himself.
In addition to his copyrighted voice, Stephen Hawking also lent the filmmakers his Companion of Honour medal and his signed thesis to use as genuine props in the film.
Eddie Redmayne met with Stephen Hawking only once before filming began. "In the three hours I spent with him, he only said maybe eight sentences," Redmayne recalls. "I just didn't feel comfortable asking him intimate questions." To prepare for the role, Redmayne found alternative methods. He lost about fifteen pounds and trained for four months with a dancer to learn how to control his body. He also met with forty patients suffering from motor neuron disease (MND), created a chart to track the order in which Hawking's muscles declined, and spent hours in front of a mirror, contorting his face. Additionally, he remained motionless and hunched over between takes, to the point where an osteopath informed him that he had altered the alignment of his spine. "I fear I'm a bit of a control freak," Redmayne admits. "I was obsessive. I'm not sure it was healthy."
Stephen Hawking said that the film was "broadly true." He then went on to lend his voice for the final parts of the film.