The Troi discussion earlier reminds me of that bit of trivia where apparently Marina Sirtis and Denise Crosby were at one point considered for each other's roles in the early development of TNG. I think Crosby ended up getting the part of Yar because she was taller, but I've always wondered if swapping their roles was a better idea in the long run.
Crosby is physically tall and imposing but she's not a very commanding presence otherwise. In the episodes where she gets to do something, she's often fairly passive, and when she talks she's really soft-spoken. Almost has a kind of kindergarten-teachery tone to her voice which just doesn't work at all for someone who's supposed to be a tough as nails security officer who had to beat people to death with her bare hands for food on a rape planet.
Sirtis, meanwhile, has a big rack and is nice on the eyes, but her body language and tone of voice are all wrong for a counselor. She's really curt, standoffish and in general comes off like she's talking down to people, even the Captain. Also when she's angry she's actually rather loud and obnoxious, she could probably have done a pretty good commanding voice if she spent a week or two with a coach. The thought of Sirtis beating the shit out of aliens of the week or threatening to shoot people is kind of laughable though, but that might be just because I'm so used to her role already.
It might also be that neither actress was particularly good at either role in the first place so it just didn't matter. I wouldn't be surprised if that's what the producers were thinking.
