While Paul Raci (who plays Joe, the founder of the deaf rehab facility) isn't deaf, both his parents were. He is a prominent figure in the deaf community as a CODA (Child of Deaf Adults) and a member of Hands of Doom, a band that performs in ASL.
A large number of the cast were hired from the deaf community.
During scenes where Ruben is struggling to reconcile his hearing loss, Riz Ahmed wore auditory blockers deep inside his ear canal that emitted white noise. He told USA Today: "I couldn't hear anything, including the sound of my own voice." After some time, he decided to forego the sound blockers and simply immerse himself in Deaf culture by communicating with the director and deaf cast almost exclusively through ASL.
Riz Ahmed said he hoped the film would encourage more hearing people to learn ASL.