For situations where focusing on the fly will be difficult consider using a wide angle lens stopped down and set at it's hyper-focal distance. Most manual focus lenses have a scale so you can see what generally is in focus at a particular f-stop. For example, a 28 or 35mm lens @f22 with the focus set at 12ft might have everything from say 5ft to infinity in decent focus or you can estimate the distance you will be from your subject and preset that but still stop down as much as possible. That way you can concentrate on your composition and not the camera.