By now, the story is legendary. Brad Pitt was still married to Jennifer Aniston when he co-starred with Angelina Jolie in 2005's Mr. & Mrs. Smith. Stories of the chemistry between Pitt and Jolie were everywhere; shortly thereafter, Aniston and Pitt split. Nearly a decade later, people still like to talk about it. But the fact is, whether the on-screen attraction between two actors is real or fabricated, it's the most necessary ingredient in any cinematic coupling. Period.
Do: Cast actors with real chemistry.