Margaret Hamilton was absolutely terrifying in her performance as the Wicked Witch. Her appearance was ridiculously scary at the time - when seeing someone with green skin was a completely new experience for cinema goers - and there are anecdotal tales that even people in the catering area on set stayed away from her out of fear. She wasn't originally supposed to look so hideous, though.
At an early stage of planning, the Wicked Witch was going to be going about her business a completely different way. She was visualized as attractive, seductive and charming, able to entice people around to her point of view with a dark charm. This idea was only changed because they wanted a clear and visible contrast between who was good and who was bad, and Glinda the Good Witch was definitively very good, so they needed to go in an obviously different direction.