To be slightly anal I thought I'd bring up the point that with this film in particular, the script adds to the score.
The reason why I say this is because Sergio Leone stated that with this film he built the film around the score composed by Ennio Morricone which came first.
The two are tightly intertwined. To answer the other poster above who said that this film would be nothing without the score. Technically that may be right as the film is built around the score and possibly wouldn't even exist without it. Or at least it would possibly be an entirely different film altogether. It wasn't a case of adding some general music to the film. It was done more the other way around. Made as much like an opera as it was a film.
Actually no, I'm pretty sure Bertoluci and co. had the script mostly done before the soundtrack.
It was just that Leone had Morricone complete the score before the filming started - so it could be played to the actors and crew and he could time the scenes correctly (instead of having to do too much post-editing). Any alterations based on the soundtrack would have been relatively minor/adhoc and the overall story and dialogue were generally as they were originally. At least, that's always been my understanding of it - that any changes would have been subtle.
The script is its own entity however you look at it. It's qualities and main narrative drive would 100% remain with a weaker score.