I was watching a bio show in Sir Anthony Hopkins the other day and he talks about why he agreed to be in The Mask of Zorro. And essentially the reasons was that he had been in several darker, more dramatic films previously and decided that, for a change of pace, that he wanted to be in a fun romp. As he put it "two hours of popcorn."
I wonder if it might not be similar for Emily Blunt? She just got done with Sicario, which is this really dark/bleak crime thriller, and Crimson Peak, where she plays a really dark character. Maybe she just wanted to have some fun in her next role or something?