The movie tells the story of the master of suspense's relationship with wife, Alma Reville, during the making of his 1960 hit slasher film Psycho.
Hopkins said he was "obviously a little nervous" about the role.
"I didn't want to hear my voice back or see any of it; I wouldn't see any playback because I didn't want to shock myself," the Welshman said.
"I saw the film when it was finally cut together and I was very pleased with it.
"It was a tough role to play, just tricky because everyone knows him and he has been such a formidable presence in our living rooms through the years and probably the most famous director of all time -- so it was daunting."
Hopkins, who used a prosthetic 'fat suit', also defended Hitch against critics that say he had a dark side.
He said: "I don't think he was a monster; he was very witty, he didn't tolerate small talk. He was a practical man and he didn't have any time for nonsense."
Mirren described her character, Hitchcock's wife Alma Reville, as the director's "artistic and domestic rock".
"People say there were four hands on Hitchcock's movies and two of them were Alma's," she said.
Asked what is was like collaborating with Hopkins for the first time, the 67-year-old said: "I was nervous, as you can imagine, he is such a hugely iconic actor and so brilliant."