Liman had been a fan of Robert Ludlum's "The Bourne Identity" since he was in high school (the first in a trilogy of books by the author, which have since been added to by seven others, by Eric Van Lustbader, after Ludlum passed away in 2001), but ended up re-reading it during the production of "Swingers" in 1996, and when that film became a big hit, the director became interested in making it into a film. The rights were tied up at Warner Bros., so Liman moved on to "Go" first, but the helmer remained interested, and soon received a tip off that the rights were about to lapse, and optioned them from Ludlum himself, flying himself to Ludlum's home in Montana to secure those rights just days after he had earned his aviation license. As of 2008, the doorbell at Liman’s Tribeca loft read “Bourne J.”; the filmmaker was no fareweather fan of the books.
A number of writers were brought on -- W. Blake Herron ("Ripley Under Ground") was the first, and received credit, David Self ("Road To Perdition") was involved at some point, and didn't. Eventually, Liman went to Gilroy, then best known for "The Devil's Advocate," but the screenwriter decidedly wasn't a fan of the source material, telling the New Yorker years later that "Those works were never meant to be filmed. They weren’t about human behavior. They were about running to airports. The filter that readers put on to read a certain kind of fiction is very forgiving.” Showing him the existing script didn't help much either: Gilroy described it as "a huge fifteen-gunmen-on-the-Metro-blowing-the-fuck-out-of-everything kind of movie." Nevertheless, Liman managed to get a meeting, and asked Gilroy what he would do, and was advised to tear up everything except the central concept. Gilroy told him, "Your movie should be about a guy who finds the only thing he knows how to do is kill people.” He was hired, and went on to co-write all three films in the original trilogy, and is directing the aforementioned new installment of the series which is mostly Jason Bourne-free.
2. The novel is wildly, wildly different from the movie.
As such, while earlier drafts had adhered closer to the Ludlum novel, Gilroy essentially started from scratch, throwing out almost all connections to the source material. Gilroy told the New Yorker, "Anything that’s from the book is in the first five minutes, in which Bourne, inexplicably, has got microfilm in his ass. Why? I don’t know! After that, when he steps off the boat, everything else is mine,” but to be fair, the connections do hang on a little longer, in terms of the basic structure; he still goes to Zurich, and takes a hostage called Marie, before moving on to Paris. But there's more action, and a different villain, with Bourne hunted by the real-life terrorist Carlos the Jackal (who was actually captured in 1994). In the U.S, one of his operatives kills a number of Treadstone employees, including Gordon Webb, a U.S. army officer who later turns out to be Bourne's brother. Our hero befriends a French military general, only to discover that his wife is a mole for Carlos. And by the end, Bourne is in New York, looking for Treadstone, confronted by Carlos and Alexander Conklin (the part eventually taken by Chris Cooper), who is a crippled friend of his, who has been tasked with cleaning up the mess. And at the end, it's revealed to Marie that Bourne is really David Webb, a foreign service officer who lost his Thai wife and children in a friendly fire bombing in Cambodia. Joining a special ops team called Medusa, he killed a double agent, known as Jason Charles Bourne, taking his identity, and later became an assassin known as Cain, created as a rival to Carlos the Jackal. Thankfully, virtually all of this backstory was dumped by Gilroy. Liman was still able to put his personal stamp on things: the inner workings of Treadstone were inspired by his father, prominent attorney Arthur Liman's experiences, of investigating the Iran/Contra affair during the Reagan administration.