Don contrasts the anime and the book.

When you make a movie from a novel, the challenge is to see how much you can jettison and still have a story that makes sense. Making a teevee series from a novel presents the opposite problem: how do you fill thirteen or twenty-six episodes with perhaps 250 pages’ worth of material?

Samurai 7 faced a similar issue, resolved by creating 80% of the series from the whole cloth and appending it to the Kurasawa's movie.