The lord of the rings has a montage right at the start of the film explaining to our audience the backstory of the LOTR mythos, this is voiced over by Galadriel, a female elf who in the book only had a small part half-way though the first book. However, as well as this voiceover, she also has a conversation with Elrond near the end of the second film. Also, Frodo in the first book was saved by a male elf called Glorfindel, while in the film he was replaced by Arwin who was only mentioned in the appendix of the book. These changes were made to add larger role for the female characters, who were downplayed significantly in the book.
The initial start of the Fellowship of the ring film was a visual sight of the battle against Sauron where as the book only told this story during a scene with Frodo and Gandalf, and the council of Elrond. This change was designed to attract attention at the start due to the action of the very large fight scene and the stakes at play.
Throughout the conversion of the book to film, the scriptwriters worked with the actors to envision what their characters would act like. (for example, John Rhys-Davies who played Gimli, ironically a dwarf character even though he is actually the tallest actor in the first movie, attempts to smash the ring with his axe without success.)
