People generally don't want to get involved unless they are "put on the spot," so to speak. If you need to get someone else to call 911 for you, don't just yell "someone call 911," because there's a good chance no one will call, because everyone is thinking someone else will do it. Tell one person, specifically, to call, and make sure they are able to pass along the pertinent info. Make eye contact, and be sure they know you are talking to them specifically.
Very true. When I took my first CPR/First Aid course, it was back before cell phones were commonplace, and the assumption was that if you were out in public, the call to 911 was likely going to be placed using a pay phone or a nearby business phone. We were instructed to tell someone to go call 911 **and then to come right back to the scene**. The rationale was that if they didn't come back, you could pretty much assume that they had just bolted, and that the 911 call probably wasn't made.