Blunt, 32, who played the role of Emily Charlton in the film, said with the eyes Hathway has, it became tough for her to boss around, reported Entertainment Weekly.
"With those lovely doe eyes of hers? Very hard!," she said.
The "Into the Woods" actress says it was "fascinating" to watch Meryl Streep playing Runway editor Miranda Priestley.
"I remember that first table read and she just submarined (everything) - she didn't even raise her voice, it was so frightening. It was just so fascinating, and I think it's actually one of my favorite things she's ever done," Blunt recalled.