There are a lot of things you can do with that lead hand. Joe louis walked you down and fired a ramrod of a jab; the potentialities of that and the rest of his arsenal were enough to keep most interested.
You don't always throw the jab from the same place, so you move it around. Drop it and bring it up from under, or get it outside his shoulder and out of his line of vision and maybe turn it into a hook. You always do something, but never do anything just to be doing something. If that makes sense.
You can do the same thing with your feet, if you are out of range. Move here and there, wave the left hand around, look busy. There's guys that have made careers stealing rounds looking busy like that.