The relationship between Paul Walker and Vin Diesel's characters was at the heart of the Fast and Furious franchise

How Paul Walker helped create a fast and furious box-office franchise - CNN.com

(CNN) -- Hollywood executives love a movie franchise: a series of films with an in-built audience who return time and again to enjoy their favorite characters, stories and themes. Such box-office gold is usually based on pre-existing properties, for example comics ("Iron Man" et al), novels ("The Hunger Games") -- even theme-park rides ("Pirates of The Caribbean").