|Photography by Dennis Davis Photography|
North light coming through a window is not very bright compared to direct sunlight. You will need to do several things to compensate for the low light levels.
Tell your model to listen to your count, and to freeze on the count of 3. They should take a half-breath, hold it, and yet have a un-posed, very natural and comfortable expression on their face. This is not an easy thing to do, and more experienced models and actors will often be better at this than first timers.
Photographers often use gold reflectors to "warm up" the photographs, and make it friendlier. This "warming" of the color temperature can also be done in photo editing software such as Adobe PhotoShop, however a gold reflector only warms the shadow side of the person, which is the bluer or cooler side anyway.