The writers helped to write the monologue and the skits. The guests were entirely on their own. I remember some nights where Johnny had a hard time with some of the guests. Anyway, Johnny would ask guests before the show what they might want to talk about (like George’s AFPA jacket) and he used that to keep the conversation going. It was shown on one of his specials how he put his show together every night. Quite different from today where guests just come on to plug something.
That may well be so, nevertheless viewers tend to underrate the role of writers in television shows and overrate the pretty face, pretty hair stars. What do writers write for game shows, for example? For news shows? Why were teleprompters invented in the first place? Talking heads are really reading heads.