A: If it were as easy as two pretty people showing up on set, getting along just swell, and enjoying a mutual attraction, then a lot more romantic movies would have a lot more chemistry than they do.
The fact that many actors who have sparked each other didn't actually get along (famously, Richard Gere and Debra Winger in An Officer and a Gentleman) indicates that mere camaraderie may have little to do with it. And the fact that major Hollywood icons who were gay in private life have joined in vibrant onscreen hetero pairings suggests that literal attraction isn't necessarily the key.
If you look at chemically dynamic twosomes (Hepburn and Tracy, Bogart and Bacall, Streisand and Redford), I'd say that what drives most of them is an honest affection coupled with -- the magic ingredient -- a competitive edge, a theatrical drive to one-up each other that translates on camera as spicy, teasing attraction.
--Entertainment Weekly, July 13, 2007