Martin, I have had weeks with as many as 50 guests and I can PREP in10 minutes if I must. Here is how:
-ask for and read bio/resume
-ask for three top questions they want to be asked
-read table of contents of their book
-then pick out one sentence that intrigues you and start with that
-have their follow up info ready
-know what exacty they want to promote
This is the beauty of a template
Not ideal-but it can be done
I've just commented on another thread about this so won't repeat what I said there, except to say that I usually get a lot of information in no more than about 15 minutes from LinkedIn profile, Web/blog "About" page. Enough for me to ask questions they might like to respond to. Some of my questions also employ a style I learned at Coachville, which is one of the Coachville "15 Proficiencies", viz. that one proficiency of a skilled coach is that he/she "navigates via curiosity": that requires listening to what is being said (and not said) and following things that "come up" or are being apparently left out. Preparation can help that, but using the process requires "in the moment" attention and seriously attentive listening, with a willingness to ask what might seem a "dumb" question.
I used to spend more time than I had hours in a day for in preparation for shows, but found that as my guest list expanded, I had to streamline my time just as Adriane mentioned. I've discovered that in my pre-interview calls, I gain much information and then use it to format my shows, along with a brief bio and a few questions. Practice, practice, practice seems to be the classic theme in life and I apply it to my show as well!