She said that she went kind of Jersey Girl on him and looked like s***, as she wore almost zero make-up.
But the 39-year-old actress then took that dedication one step further by embarking on a strict diet and losing about 15lb for the film, as she thought that it would add to her sense of being a little lost or sadder.
Mendes, who is dating actor Ryan Gosling, also confessed that she has gained most of her weight back as that was a lot for her to lose and that she didn't like the hard and toned look.
Mendes said that being sexy is just one component of who she is - it?s a thing that she can tap into, just like she can tap into her funny, quirky or her dramatic side and it's not what she is.
She asserted that her looks had probably caused some directors and studios to shy away from her for certain roles.
She added that she is the one who is responsible for whatever image of herself she has put out there, as this business often wants to sexualise women and she understood that but at some point she got tired of being always thought of as sexy and allowed that description to define the kind of work she was doing.