The old westerns show the gunslinger "fanning" the hammer - generally very showy but not conducive to accuracy! With the triger held back, the other hand simply sweeps over the gun, pulling the hammer back and then releasing it to swing forward again - "BANG!"
Some pump shotguns can do the same, holding the trigger down and firing on each stroke of the pump.
Many revolvers and pistols can fire if dropped in such a manner the hammer (down on a loaded chamber) is able to hit the firing pin and fire the gun.
But, in steps Mr. Gun Safety Guy, and has them redesigned to help prevent such activities.

Modern gadgets like the "Transfer Bar Safety" and various firing pin safeties are intended to make this less possible.
As far as tripping the hammer putting the gun into the holster - no, not enough travel in most cases. Some Glock accidents have happened when the owner holsters the pistol with his finger IN the trigger guard - a No-no!
Hope all this helps . . .