Here is what a proffesional actor says in an answer to [a related question on Quora](https://www.quora.com/How-do-they-stop-people-from-breathing-in-movies-when-they-are-dead):

> I am an accomplished actor and have had to do scenes where I have
> died, the first thing to do is to prepare for the shot, the director
> will tell just how long the camera will be focused on you. Before the
> shot you will be taken to the Make-up department to be made up to look
> like your are dead if this to be a scene that you are in a coffin at a
> funeral. As in example when I did an episode of a murder show on ID
> Discovery and played a murdered victim where the camera would be
> focused on me for a period of time when the director was ready for the
> shot He would ask for a hand signal when I was ready in which I would
> take a deep breadth and then hold it and hold my position the director
> would the take the shot and film for about 40 to 45 seconds and then
> cut. In some cases the editor can also cheat a shot using a still
> image of you in position and your still image is matted into the scene
> where other actors are moving but you are not because your footage
> that was shot was now placed into the scene as a still shot. Another
> thing the is done with some actors when their death involves the eyes
> to be opened and since you are not really dead your eyes will
> sometimes blink while some actor can hold this for a few seconds,
> what’s done then is that the rest of the shot is live but the editor
> creates a still of only your face and then matts it back on to your
> face as a layer. There are many things that can be done in post
> production to accomplish this effect to make it look as real as
> possible.