People tend to walk in a rhythmic manner. Wild critters are arrythmic. They take a step or two and stop to nibble or browse and then take another step or two.
Even a short pause, every two steps, will break any rhythm and fool critters into thinking another harmless critter is wandering about.
Generalizing, since deer ain't read no books, deer tend to bed down where they can smell Bad Things coming from behind them, and see/hear Bad Things coming from downwind. E.g., the downwind crest of any sort of ridge. Also, they'll be near any sort of saddle in a ridge, so they can avoid being skylined when they take off, and have cover if in relatively barren country.
"Real" bucks will generally head out upwind, and uphill. Does and little bucks will go most any old how--which means, if you see does running, don't watch them; Bucky is elsewhere.
Back to the running deer thing: It's all integral calculus of your mind. A running deer is about a 30mph target. That means at 100 yards, and a 90-degree path, four feet of deer travel for your bullet to intersect in that one-tenth of a second of bullet travel. So, hold two or three feet in front of his nose--ignore the up and down; it won't matter. If he's quartering, less lead. If he's much over 100 yards, don't shoot...
If you're hunting open country where it's a matter of shoot a running deer or forget eating venison, use a bit more gun than otherwise. A bad hit from an '06 is more likely to put a deer down than a bad hit from a .243, for instance--and your second shot should then end it.
But first, practice offhand shooting--a lot!
, Art