How is Fahrenheit 451 the novel similar to the film version? Why did the film keep certain aspects of the novel the same? how is the novel...

I haven't seen the movie in awhile, but no one else has helped you yet so I'll give it a try.

First, producers, writers, and directors normally try to keep a film as similar to the original book as possible.  That would be the norm. So the short answer to the second part of your question is:  because they could. 

Now, just to clarify that, novels and films are two totally different mediums, so most of a novel is not transferable to a film.  Most of a novel is left out of any film because the content simply will not work in film. 

Concerning the first part of your question, the main characters remain in the movie:  Montag, Clarisse, Beatty, and Millie.  (Clarisse and Millie are played by the same actress, by the way:  Julie Christie.)  Firemen still burn books, the woman burns herself to death after she refuses to leave her books, Montag still undergoes transformation, etc. 

In other words, there are many similarities between the novel and the movie.  Perhaps one stands out:  the setting is depicted in the movie much as it is presented in the book.  Everything is barren and colorless, lifeless.  Millie even has her TV walls to obsess over.  The setting is joyless and changeless in the movie, as it is in the novel.

One contrast also stands out between the movie and the novel, however:  Clarisse is definitely not sixteen in the movie.