Answer to the question posted in the subject line----when it is a Presidential "debate." After listening to the debates of both parties this year we should label these exercises for what they are--"getting my talking points out no matter the question asked." Ignore the question if your answer isn't what you want to address and give the answer you want. Often the person asking the question doesn't follow up and make the candidate answer the question asked instead of their talking point. Even worse is when the moderators stop the other candidates when they challenge the answer given. Often that is the only time stupid answers are ever challenged.
Having done debates in high school and college I know what a real debate looks like. First, you better come prepared to back up your statements with facts that you can reference from a credible source. Just saying something as a fact doesn't proven it just because you said it. Donald Trump is the best example of this point. Second, equally important in any debate, is being prepared to counter the arguments of your opponents. Points scored in your counter arguments count as much as the points you score in making your own arguments. This requires you to examine the weak points in your arguments and be prepared for your opponents counter arguments. A technique that is useful in any debate preparation is to argue both sides argument to refine your points and counter points. Lastly, a moderator isn't there to ask the questions but to moderate the back and forth between two debaters. I realize this is difficult with the large number of candidates in these debates (especially with the Republicans) so let's just call them something else than a debate.
Anyway, most people watch these debates to see how their candidate looks and reacts than what they have to say.