What should you actually do if someone collapses?

If someone collapses and is unresponsive and not breathing normally, the sequence is: check responsiveness, call 911 or direct a specific person to call and to find an automated external defibrillator, then begin chest compressions immediately. Place the heel of one hand in the center of the chest with the other on top, keep your arms straight, and push down at least two inches at a rate of 100 to 120 compressions per minute, allowing full recoil between compressions. The tempo of Stayin' Alive is the standard reference and is genuinely useful. Do not stop until emergency responders take over, an AED is ready to analyze, or the person shows signs of life.

For untrained bystanders, hands-only CPR without rescue breaths is what the American Heart Association recommends for adults who collapse suddenly, because interrupting compressions reduces effectiveness and because reluctance to perform mouth-to-mouth causes bystanders to do nothing at all, which is by far the worst outcome. Survival from out-of-hospital cardiac arrest is low and bystander CPR roughly doubles or triples the odds, which makes imperfect action from an untrained person vastly better than waiting. Infants, children, and drowning or overdose cases are the situations where rescue breaths are more important, since those arrests are more often respiratory in origin.

Two things worth knowing in advance. Automated external defibrillators are designed for untrained use: they give spoken instructions, analyze the rhythm, and will not deliver a shock unless one is indicated, so using one cannot hurt the person and early defibrillation is the single strongest predictor of survival. And Good Samaritan laws in every state provide legal protection for people who render reasonable aid in an emergency, which addresses the liability worry that stops some bystanders. A few hours of certified training through the Red Cross or American Heart Association makes all of this considerably more likely to happen well.