After Steve Carr decks Robert McCay in the restaurant, Carr is already pointing at McCay in the long shot, but on the cut to the close-up he's just raising his arm to point at him.
If someone else had exchanged the blank for a live round, the state would drop the homicide charge against the defendant in favor of a reduced charge of manslaughter, due to pointing a firearm at another and failing to verify a blank round was loaded.
The plot revolves around the insertion of a live round into a revolver used on a movie set to fire blanks. Blank-firing guns cannot fire live rounds without significant adaptation, so this would not be possible.
The $150,000 bail is a little light in a capital murder case (even in 1986 dollars and even though McCay isn't much of a flight risk), given that he's a big movie star with lots of monetary resources at his disposal and in most cases bail could be obtained for only 10% to 15% of the bail amount (just $15,000 to $22.500 in this case).
The suspect tells Mason he is not a murderer, however gross negligence such as pointing a firearm at another coupled with failing to check that it did in fact contain a blank round constitutes manslaughter which means he was responsible for their death but was not intended.