All employers want to know that you really want to work for them. That doesn't mean you have to jump up and down, smile all the time or be effusive. It just means you need to look them in the eye when they talk to you and listen to what they say. On the job, you have to show some interest in learning how to do your job the best way you can.
Every job requires enthusiasm if you're going to do it well and not hate your life. Even more somber jobs like funeral directors require some interest in the field. Some jobs are quieter and more serious - but you still have to find interest in what you're doing, or you'll spend 40+ years in a rut. That's not fun.
well, the guys who sell the food in walmart seem to be in bad moods all the time, at least in my experience. You know, those where you get the rosted chicken, mash potato and all that