Here are three key differences:
1. Movie trailers are effective because they are shown before another movie which has been deemed to appeal to the same demographic. Book trailers aren't shown in books deemed to appeal to the same demographic. They are usually on YouTube, an author's website, a book page, a publisher's page etc. They, therefore, have limited effect if you don't already know and like the author. You need to be directed to them in some way. They aren't put in front of you just as you're about to read another novel, the way movie trailers are put in front of you just as you're about to see another movie.
2. Movie trailers can take all the images they need from the movie itself. Making one involves selecting, assembling and contextualising existing images. Book trailers, however, need to create these images from scratch, and the danger is that this process can lead to the opposite effect you are trying to achieve. Reading (unlike watching movies) is about getting people to create their own images in their heads. Providing them with images can destroy the magic.
3. Movie trailers are the end result of the work of vast teams of movie-making professionals and feature experienced actors of the highest calibre. Book trailers don't have these advantages. Although some obviously have quality actors and high production values, they simply don't have the resources behind them that movie trailer producers have.
To try to bring the differences into sharper focus, let's compare the movie trailer for the recently released 20th Century Fox film, The Book Thief, starring Geoffrey Rush, with two book trailers for the Markus Zuzak novel on which the movie is based.
Movie trailer for The Book Thief
Book trailer for the YA edition of The Book Thief
Book trailer for the adult edition of The Book Thief
So, how do you judge the success of a book trailer? The obvious answer of the number of views isn't the whole story. I'll have a closer look at this in part 2.