Kate Mulgrew, who played Kathryn Janeway on Star Trek: Voyager, was not the first pick for that role. In fact, Mulgrew only got it after another actress quit midway through the filming of the first episode. Here's a scene from that episode with that actress, Genevieve Bujold.
Do you think that she would have made a good Janeway?
I found this video at list of fourteen facts about Star Trek that you might not know, which you can view at the link.
Link via blastr