These are head concepts for the three main characters in XCOM 2: Dr. Tygan, Engineer Shen, and Central Officer Bradford.
For these types of studies I often go back and forth between sketching, painting, and sculpting in Zbrush. Most of the changes and feedback after this stage happen either in Photoshop or directly on the final 3D sculpt. I always enjoy doing characters; it gives me the opportunity to think about their personalities and stories, especially with the face.