The writers had to make the roles on the females bigger and more important in the film. They did this because there were to many males compared to females and it would have made an unfair balance. This as you can imagine is very difficult to do for obvious reasons such as fitting extra characters into scenes that aren't meant to be there, making them important to other characters and giving them a role that could fit into the story. They also had to take out certain characters in the film, the reason for this is that some of the characters in the book didn't fit with the story they were trying to portray.
Writing an exposition can be very hard. The writers will have to condense a big part of the book into a few pages of script, it has to make the film move along quickly but also include all the information that the book did. Often they will have to make up scenes for the film that never happened in the book and make sure they link into the scenes that did. But without changing the story to much.
The writers would often get together with the actors and gather ideas from each other. This could help with things that fans would be expecting to see in the film that happened in the book. Such as where Sam holds Frodo's hand whilst he is in bed. This is a good technique for the writers to use because they get a wider range of ideas.
There are different styles of script writing. One completely different to movie script writing is writing for a series. A good example would be 'The Big Bang' when writing scripts for this show the writers don't only have to think about the audience at home but also the live audience on set. The live audience need to be amused, making them laugh is very important. Whereas with the film making the audience feel attached to the characters can be a bigger priority. Another thing is that the fans of Big Bang theory never know what is going to happen in the next episode, this gives the writers a lot more flexibility and lets them almost write whatever they like, from introducing a new character to having a character go to hospital. That I think is easier than writing a film such as lord of the rings where many people have read the book and know what to expect, and if they think it doesn't live up to the books reputation then it wont be a successful film.
