Gervasi said that he was immensely proud to have cast Johansson as Janet Leigh and Biel as Vera Miles in his movie about the controversy surrounding 1960 film "Psycho", reported Digital Spy online.
"They are amazing. They are both extraordinarily beautiful and ridiculously talented, but they were very professional on set," the director said.
"Scarlett took it incredibly seriously. She read all the books, she called Jamie Lee Curtis - Janet's daughter - she got all her facts right, she researched what she wore, what she would be smoking on set, like she knew every biographical detail," he added.
The filmmaker also credited Biel with bringing "a kind of vigour" to her portrayal of Miles.
"Obviously they are big stars and extraordinarily beautiful, but as actresses on the set, they couldn't have brought more," Gervasi said.
Johansson recently said that it was a challenge to play an iconic film star such as Leigh.
"Hitchcock" is now playing in the US and hits the UK on February 8.
