Interviews are a serious business, but that's no reason not to smile. Smiling will show that you're relaxed and confident, as well as generally pleasant. No one wants to work with a dour sourpuss.

Keep in mind that there's a difference between a forced smile and a natural one, so as much as you can, try to relax and smile as naturally as possible. A natural smile involves all your facial muscles, including your eyes, while a forced one will move your mouth but leave your eyes unmoved. Either that or you've overdone it with the bot-ox.