It might be different now, but I'm pretty sure back in the Bob Barker era contestant selection wasn't random. The entire audience had to introduce themselves to the producer one by one and answer his questions about where they were from, what they did, and if they were there with anyone. The people selected were fairly attractive and charismatic. The selection also leaned toward people who came with larger groups. I…
Before the show, producer Stan Blits greets every member of the audience while they're waiting in line. He chats with each audience member (for about 20 seconds) to gauge their liveliness & enthusiasm. As he's chatting with each person, he gives a subtle hand signal to an intern who is standing behind him with a notepad. For each audience member who he thinks might make a good contestant, the intern writes down the person's name on the notepad.
After the audience is seated, he continues to refine the list by standing on the side of the stage and observing the potential contestants in their seats. He checks to see which ones are still energetic and cheering while they're sitting in the audience too.