Window light is not all the same when it comes to shooting portraits. Having your subject face directly towards the window gives you a very flat light, similar to your on-camera flash. In the second photo, the subject is looking straight at the window.

In the first photo, the subject is about 45º from the window.

Fujifilm X-E2, Nikon 85mm ƒ/1.8G, Nikon AI Mount Lens to Fujifilm FX Mount Camera Adapter, ISO 5000, ƒ-wide open, 1/500

Notice how the face has more shape from the light coming from the side rather than straight onto the front?

Fujifilm X-E2, Nikon 85mm ƒ/1.8G, Nikon AI Mount Lens to Fujifilm FX Mount Camera Adapter, ISO 5000, ƒ-wide open, 1/500

Here the window is directly behind the subject. The room is kicking the light back to help fill in the shadows. You are very close to creating a silhouette like the one below.

Just because the subject has window light doesn’t mean it is excellent. It is the direction of that window light that makes a HUGE difference.

I suggest doing some test shots where you shoot, moving the subject and you. Then before you do the actual photos, pull them up on your computer and examine them. Then pick the few angles you liked the most, recreate those lighting conditions, and shoot away now looking for great expressions.