It was a revolver though wasn't it so checking if it's a blank vs live would mean opening it, taking out every round checking them and reloading it.
No, it wouldn't, they're contained in a drum and easily observed.
Admittedly easier than having to unload a magazine to check each round there.
Why not check when loading the magazine in the first place?
As they will be firing blanks past cameras at times so "just checking a round is there" doesn't do much as a round is meant to be there
That's kind of missing the point - he was expecting the firearm to be "cold" not with rounds of any type, i.e. he wasn't expecting it to fire! (not that he should have been pointing it at anyone regardless).
So what do you do when there is meant to be a blank chambered?
You load a blank??? I'm really not sure why you're struggling with this or trying to make it difficult as though no one who has used firearms has ever considered things like weapons saftey before?
Re: the other post and a live round being mixed up in among a magazine of blanks - that's a different issue but rather rarer, the issue with mixing live and blanks is more likely to be something dumb like one being in the breech or someone having one in their pocket/picking one up off the floor at night and putting it into the top of the magazine. (if your rifle was previously made ready and you then need to make safe or strip it down and clean it then a round is ejected for example).
You solve that by being rather careful with loading them, not much excuse on a movie set, you can do it in plenty of light, you're rather less likely to accidentally happen to find a live round among your possession or on the floor in the pitch dark etc..