I have a silly question about villager skills. When I take a villager (say a scientist) and put him/her to work in another area (like farming) will he/she gain skills in that area automatically or do I have to change their skill preferance first?

The preference box affects a villager's choice of what to work on when they are deciding what to do next. It allows you some control over their choices, but doesn't directly affect their skill levels. A villager will gain skill any time he or she is successful at any skill-related task (e.g. building), whether initiated by you or by the villager and regardless of the villager's existing abilities.

Thanks, Barbara!
I thought I was probably wasting a lot of time by changing their skill everytime I wanted them to do something differant temporarily, but i wasn't sure.

Before I quit my game I always select at least 2 villagers as doctors and I put them on a task as Farming or Research. And the tomorrow, when I go back to my game, I can see their skills in these tasks have increased!!