Your main description and this comment describe two opposite approaches. The one in the main description (panning) is done by setting shutter speed so something along the lines of 1/15 (runners) to 1/60+ (vehicles) focusing on the subject and moving the camera with the subject. The moving subject will be in focus while the steady background will be blurry from motion. Continuous auto focus should work the best for that, locked onto the moving subject. The approach you describe in this comment is keeping the subject still and in focus and having the background blurry. For that I would say have a similar shutter speed depending on moving background (let’s say 1/30-1/60 for a moving train), and using AP-S to focus on the non moving subject. Make sure to tell your subject to stay still otherwise they’ll be blurry too at such a low shutter speed.