That's a given, surely? In such circumstances, one would switch on one's telephone and call an ambulance, the projectionist would stop the film, etc.

In fact, during a preview screening I attended for The Passion of the Christ about a month before the release (it was for church pastors), the lights suddenly came up halfway through and the film was halted, because someone in the audience had a heart attack or stroke (I can't remember which). They carried him out on a stretcher, then the lights went down and the film continued. Given the controversial and violent nature of The Passion of the Christ, the incident merely added to the feeling that perhaps we might not all get through the film alive.