Say you’re an actor, and I hand you a revolver, assuring you that it is not loaded. The scene it’s involved in requires that the hammer is already pulled back (as the character in question is threatening someone at gunpoint).
Should you, the actor, check the chamber? With the hammer back and the cylinder locked, doing this would require a complex maneuver of blocking the hammer with your finger, PULLING THE TRIGGER, and then rotating the cylinder to look at the one that was chambered - then rotating it back, and re-cocking it.
Now imagine, being an actor that is a novice with revolvers, you mix up which spot you’re meant to block with your finger. If, as you suggest, there is any chance at all that there’s a live round in the chamber, aren’t you introducing further risk with this maneuver?