Francis Ford Coppola improved tremendously the shlocky source material by Mario Puzo, and actors like Marlon Brando, Al Pacino, James Caan, and John Cazale did the rest.
The film that birthed the concept of the blockbuster was based on a deeper, denser novel by Peter Benchley that director Steven Spielberg chiseled down to craft a suspense thriller that kept viewers on edge.
The Robert Zemeckis film, whose script was adapted by Eric Roth from a novel by Winston Groom, truncated and deleted elements of the source material to make it more believable and thus more enjoyable.
The Nicholas Winding Refn film was based on a book by James Sallis which Hossein Amini made sharper, sleeker, and considerably more brutal to fit the demands of the cinematic experience.
Abhishek Kapoor did a smooth job of adapting Chetan Bhagat's most true-to-life novel, and had the advantage of the collective talent of Rajkummar Rao, Amit Sadh, and Sushant Singh Rajput to drive his film.
While not the tallest of orders, Abhishek Varman's film works far better as a whole than the Chetan Bhagat novel it is adapted from, what with the novel being too filmy for prose but the format of the big screen working in favour of the story.
Meghna Gulzar's adaptation of the Harinder Sikka novel deviated from the source in places, but also had emotional heft and spirit that the book sorely lacked.