So when you see those "quickie" comments between segments on say, for example VH1 - this is how they do it:
It is obviously most effective to get all the lines recorded prior to editing the entire episode of a show. For example, if a show, and we'll just use the example of VH1's "Best Week Ever", needs quickie commentary during various segments flashing across the screen, it makes sense to have the various hosts or hostesses simply record the commentary in one sitting. This saves time, reduces cost and sometimes can get outright silly. Of course if you were simply a passer by and saw this process in action, you would wonder, what the heck are they doing? And just why is she sitting in front of that ugly green background? This is just another reason to make good use of chroma key technology.
One never knows 100% for sure what will be running in the background as a clip, so it's easier to work with "raw" green screen footage for this purpose as this allows any footage to be inserted after the fact.