They study those the interview beforehand and take on notes on things to ask them, they also through practice have an arsenal of overused questions for whatever interview situation might be. And when neither of those aren't enough they improvise but that is not really typical of mainstream media they stick pretty much to a script, you know they have a screen they read off of it is written by groups of writers they are probably listed in the cast and crew section if you wiki any particular news group.
Most news interviews have a "practice" session ahead of time. Also, experienced news people know how to ask questions and think on their feet when needed. The news business requires mental dexterity and an ability to think quickly.