When a big movie comes out, I like to buy the disk rather than just watch it on Netflix because I want those extras. I think I have watched the extras more often then movies themselves. This certainly was the case with The Lord Of The Rings trilogy. Cloverfield was a good one as well.
Fight club had interesting extras. They showed how they had to overcome many problems in making the movie.
The recent Star Wars movies have lots of great extras. The "Episode 1" extras start with George Lucas sitting at his desk writing "A long time ago, in a galaxy far far away..." (I wonder if he realized what his writing of that sentence again really meant for so many people.) The extras ended with people running into the theater with tickets.
Lord of the rings showed much about the actors throughout the whole movie making proccess. You can see the hows and whys of each character cast. Then, see how the actors attempted to portray each character. On the third movie, watch what happens when they wrapped. It was all there.
I play these extras when I need inspiration to write or edit. If you are interested in film making, go through your movie collection or need that occasional bit of inspiration, lay those extras for insight, and inspiration.