For the last several weeks, CNN producers sorted through 4,927 questions -- previewing more than 3,000 -- and narrowed them down to several dozen by Wednesday, said CNN Political Director Sam Feist.
The main criteria: the questions must be "important in the Republican primary," and illuminate differences between the candidates, Feist said.
"We want to have a really diverse mix of questions. We want to have some straight forward questions about policy differences from the candidates and also want to have a personal story when it makes sense."
Republican candidates were reluctant to participate in a similar debate, concerned that the tone of the questions made light of the office. So producers agreed that the chosen questions would have the appropriate tone.
CNN producers selected questions from vloggers who offer geographic diversity, different diverse and ages, Feist said.
"We will go into the debate with a lot more questions than we ever expect to use," he said. "A question that might make sense before the debate starts, might not make sense after the debate starts."
Host Anderson Cooper will carry a set a cards listing the questions and he and the producers will decide which video to queue up as the debate proceeds. "These debates take on a life of their own," he said.