My homework for my photography class was to take a few portraits in natural light and with flash. I asked my gorgeous coworker if I could take some pictures of her since pictures of pretty girls always seem nicer. 
It was my first time taking pictures of a non-family member or close friend. I got a few decent ones as we were both cracking up so I got some nice natural smiles. I never knew how hard and akward this picture-taking business is.
Now in order to get other expressions, I was just wondering what you tell your models. Do you tell them, "Okay, now look serious."? "Now mad!" I remember seeing one photographer on tv years ago tell the model, "Pretend you see someone far away and you're trying to recogize that person to see if it's your friend." Or do you just let your models make whatever expressions they want?


