On the way home this evening, I was listening to NPR's "All Things Considered." There was a story about movie roles  that were written for a specific gender, but in the end were played by someone of the other gender. Most were male roles switched to a female. I can't imagine "Alien" without Sigourney Weaver as the lead.

Blogger Alyssa Rosenberg  points out that roles that are switched from female to male are usually smaller. (And the characters were usually lawyers...There's a thesis in there somewhere.)  I also thought her comment about the settings was important. The female leads are in charge, but they are in small, confined spaces. Check out her pop culture blog.