How do talk shows work?

I.e. do shows approach actors etc. to come on their shows or do their agents agree it with show producers e.g Jonathon Ross coz they want some publicity for their client? Do the guests get paid @ all or do they do it for free to promote themselves? Would an A Lister like Julia Roberts get paid a lot more than say Jade Goody to go on a show? Is it all rehearsed or all done on the night?

Well I been told once that famous people usually go on chat shows to promote a book or film. There agents do this for them. It works both way the TV host wants the rating up by have hot star on there show, while the star has something to promote or flog somthing i.e new film, tv show, new book etc. Most shows are pre recorded shows.

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