Redmayne brukte et halvt år på å studere Hawking's liv, hvor han blant annet så på alt intervjumateriale han kunne finne. Mars uttalte at det Redmayne måtte gjøre ikke var særlig enkelt. "He had to take on enormous amounts of difficult preparation as well as embracing the difficult physicality of the role. It's not just doing a disability. It's actually charting the course of an illness that erodes the body, and the mind has to project out from that erosion," sa han. Han la til at Hawking hadde vært i støtte for filmen, og at "[Hawking's] response was very positive, so much so that he offered to lend his voice, the real voice that he uses. The voice you hear in the latter part of the story is in fact Stephen's actual electronic voice as he uses it".