The Gossip Girl actress debuted the new style at the Time 100 Gala in New York on Tuesday, where she told People that the transformation has helped her delve deeper into the role.
"I thought [my character] needed red hair," she explained. "It helps when you're playing a role, when I don't feel like myself. And I don't really feel like myself with red hair."
Lively will play a confidence trickster who takes a teenage girl under her wing in the movie.
She went on to confess that she is fond of the look, adding that she feels "like Ariel", the lead character from The Little Mermaid.
"I love it," she added.
Lively previously revealed that her bodyguard insisted that she become a brunette for her role in The Green Lantern.
She previously credited her mother with helping her develop her fashion sense.