The British actor, who plays Sir Lawrence Olivier in "My Week With Marilyn," said that she had mystique in a pre-internet age where the power of the movies gave her an extra-special allure.
"Someone like Marilyn on the movie screen was as far away and exotic as you could be. When she arrived at Heathrow in 1956, the traffic stopped because Marilyn arrived," Branagh said. "Not only did she come from Hollywood, but she brought sex. Britain was being taken out of its postwar austerity, and Marilyn was a symbol of this new freedom."
"My Week With Marilyn" documents the time the blonde bombshell spent filming "The Prince and the Showgirl" with Olivier.
Branagh said the shoot was tense because Olivier and Monroe were often at loggerheads -- but there was no denying the attraction of his leading lady.
"She was such an enormous movie star at the time, and she was someone that everyone wanted to know about and, perhaps, everyone thought they knew, and that still seems to be the case," he added. "Everyone wanted to fix her. She was unutterably thrilling, shocking to some, outrageous, but she could not be ignored. She was an enormous star."
The task of playing Monroe fell to the actress Michelle Williams, whose performance impressed critics.
"So many people have so many ideas about who Marilyn was. At a certain point, after you've done your research, you have to filter all the noise out, and you have to pin something down and say, 'OK, this is my Marilyn,'" Williams said. "This is how I see her, how I interpret her, and I hope I do it justice. It's a personal relationship."