I'm a 2nd grade teacher, so it depends. When I'm doing lessons, the kids seem to learn more when I'm in my uber-ENTP happy, inventive, spur of the moment mood. You know-- when I can throw my lesson plans out the window and say "We're not gonna read our Science books! We're gonna.... Go outside! And... I'll be the Sun and you can be planets and you can orbit and rotate around me! Yeah! That'll be fun!" Of course, other times I've got to be all serious to get things done because too many fun activities in a row, while I'm happy with them, tend to make the kids too crazy. In that case I've got to calm myself down.
My summer job, otoh, I need to be serious most of the time. Our job is to help people have fun, so we're supposed to be outgoing and fun-loving, but we've got to keep people safe first and foremost. Sometimes I have trouble balancing those things. I get on the ego high BusinessMan was talking about in another thread and I'm walking around interacting with everyone and having a good time and feeling like the most awesome guy in the world until the end of the day comes and I find out I was so busy running around being king that I forgot to do some essential thing and get chewed out by three different people. That'll bring you down off your ego high pretty quickly. So with that job I try to first get everything done and then I let myself get a *little* fun. But I really have to keep myself in check or I start forgetting things and everything goes to hell and I've got a manager twice my size crawling up my butt. So yeah...