Like in the franchise itself, history has been changed, and the original script for Terminator: Salvation ended up getting gutted.... The biggest change came when [director] McG flew to the UK to talk to Christian Bale about starring in the fourth Terminator movie. The director wanted the Batman star to play Marcus Wright, the cyborg protagonist of the script. But Bale focused on another part: John Connor. The only problem is that John Connor had about three minutes of screen time in the entire film; most of Connor's moments were played offscreen. In the original script John Connor was the secretive leader of the Resistance. He lived on the HQ sub, and almost no one saw his face, so as to keep him hidden from the robots. Connor made radio addresses and existed as a legend for the fighting men and women of the Resistance, but in the original script Connor didn't show up onscreen until the last minutes of the movie.... [Bale] had something else up his sleeve: massive rewrites to beef up the John Connor role....
The script that ended up getting shot never quite finds anything for John Connor to do. If you were to remove Connor from the film, relegating him once again to radio voice over, almost none of the film's plot would be changed. It's likely that the new Connor scenes were the work of Jonathan Nolan, who did do a lot of writing on the film, but who was denied credit by the WGA. The reason would be that all of the work Nolan did was cosmetic - adding Connor scenes that had no bearing on the film's structure or plot.
There's a good deal more for any readers interested, including the considerably bolder (though highly problematic) ending that also wound up getting scrapped.
I found this news interesting in part because I had a short interview a while back with Pete Docter, director of Pixar's Up (my review will be posted Friday; short version: terrific), and one of the topics we discussed was the far greater creative control that writers and directors have in animation generally and at Pixar in particular. Accomodating divas who want the entire film rewritten so that the tiny role they're interested in becomes the lead just isn't a part of their portfolio.