Actors are much better coached in stance and movement and the general gross body movements with a firearm in their hand, but they never seem to really know that to do with their support hand on the pistol. Sometimes it's a glaring teacup Weaver, but most of the time the support hand is just sort of along for the ride.
As y'all know, if you can get both hands on the gun, it's the support hand that does most of the grip work.