Decocking

[Yournamehere]

The reason people do this is because that's how you had to do it back in the days when there were single actions without decockers or double action revolvers. In other words, like Nyles said, it's force of habit. For someone like an actor, though, it's probably the fact that they don't know there's a lever there that can decock the pistol, and just go on what they've learned, which is usually old world knowledge.

That being said, if it has a decocker, use it when decocking the pistol. If it doesn't, you should manually lower the hammer very carefully if necessary, pointing the gun in a safe direction. Most of the time, a manual hammer lowering is needed for a single action pistol like a 1911, wherein you should just put the safety on and keep it in Condition One until it's unloaded.