It's odd that a film completely driven by movie stars is being advertised with a poster that features blank white silhouettes where those movie stars should be. Tom Cruise and Cameron Diaz are the stars in this case, bringing nothing but a generic all-purpose movie star quality to the proceedings in Knight and Day. Maybe that poster is just a tip-off -- a visual reminder that Knight and Day is a cookie cutter action-comedy, and that anyone with the right amount of box office pull could fill these blank roles.
Diaz is June Havens, Romantic Interest. There's no characterization for Havens other than being beautiful and a little clumsy ("clumsy" being the most popular studio shorthand for making her relatable to women everywhere). She twice bumps into Roy Miller, played by Cruise (also a non-character, Super Spy), while boarding a plane, and ends up having a lengthy conversation with herself in the plane's bathroom mirror over whether or not she should make out with him (because, hey, he's Tom Cruise). While this is going on, Miller is killing everyone on the plane with Ethan Hunt-style lethal precision. It's a nice intercutting of sweet and sour, and possibly the last time the movie feels legitimately dangerous.
Soon, Havens is being targeted by government operatives who are chasing rogue agent Miller, and Miller keeps swooping in at the eleventh hour, drugging Havens (in what feels like an unintentional running gag), and moving her along to the next big international location so that something can explode. They careen through explosion after explosion, bullets hitting every single object around them but them, all in the name of saving a battery prototype called Zephyr from falling into the wrong hands (the wrong hands belonging to a directionless Peter Sarsgaard).