how would you go about auto focus when creating film like content?

If I want to record myself for some film type of videos with the back facing camera of a iPhone 15 pro, how would I be able to see if the shot in terms of lighting and auto focus looks good? lets say for example I set up all the settings before I jump infront of the camera, wouldnt these settings all change when I go infront? or if I lock them in then it wouldnt auto focus on me properly?