No, not really. Graham Yost came up with the story idea after watching a Kurosawa flick, as he says. He got the structure and the characters in place. Fox liked the script, and gave it to Whedon to doctor up. Whedon redid a large amount of the dialog and changed the Alan Ruck character from being an ass into a dorky tourist, and made Annie less comedic. The script then went to arbitration and Yost got the credit after some backroom politicking (according to Whedon) although Speed, at one point, in anticipation of how they THOUGHT the arbitration would go, had a poster crediting Joss Whedon alone with the screenplay. But the Writers Guild, upon learning that Whedon basically, when it came down to it, only did a dialog polish and a few character tweaks, decided the script was still mostly Yost's.

Carrie Fisher did nothing on Speed.