"But what is amazing about her is she absolutely never plays with that. Scarlett doesn't care. It's about the part."
Lucy follows a student who is accidentally caught up in a dark deal and whose brain power is unlocked to a superhuman degree, turning her into a warrior who has evolved beyond human logic.
Speaking to the Evening Standard, Luc added: "She is very sweet. She is almost acting like a totally tough man. The beauty just comes by itself. I can see it through the lens and I can use it but she doesn't play with it at all."
Scarlett Johansson filming Lucy with Luc Besson
Scarlett, who is also the star of The Avengers films as Black Widow, is expecting her first child this year and is rumoured to be getting married on Saturday.
It is not known where the star, 29, and her journalist love, 32, will wed but it is expected to be either in Romain's native France or the Hamptons, where they have spent most of the summer together.
The Under The Skin star talked wedding plans in the May issue of Glamour. "We're just going with the flow," she said. "I've never been one to do a full-on themed wedding. I don't care about that stuff."