Oh, I just realized another place where they gave a good reason for something that was never in the books. And I had to think about the movie in retrospect in order to notice this... Oh this is clever....

In the book, the name "Ford Prefect" is a throwaway joke. He says something along the lines of how Ford, in his initial research about Earth, somehow thought it would be an inconspicuous name. Never really detailing why. I mean, it's funny because Ford Prefect is the name of a model of car in England (or at least it was around the time Douglas wrote the first radio play), but we're never told why Ford would want to name himself after a car.

In the movie, there's an actual scene where we see Arthur and Ford's first meeting, as a flashback. And Ford's explanation of that moment is (now that I think back on it) the reason he chose the name "Ford Prefect". It's all so clear now.

Wow, that was damn clever of the writers. I wonder if that one was Douglas' idea.

There's a bunch of other places like that in the script, where they've taken things and connected them up like that. In that example, the reason Ford and Arthur met is also the same reason Ford chose the name he did. There's a bunch of other examples just like that, where there are connections that never existed before. And when you see them in the movie, they seem like such logical connections, like they were meant to be there all along.