Firstly, while reading a book we visualize main characters in our way. Sometimes we cannot imagine various characters or surroundings where action takes place. In this case we cannot fully understand some writer’s ideas.
Furthermore, it is amazing to see how director sees characters. There are lots of different nuances that we cannot find out while reading a book. When you watch Harry Potter and his friends on TV screen, his magic adventures become so wonderful that you cannot turn your eyes away from the action.
On the other hand, some events are not showed in the film as it lasts only a few hours. In this case sometimes we can see just main actions so the story is not told us with detailed descriptions.
In conclusion, I think it is not bad to watch a film version of favourite books. We can really find out new qualities of characters and see at the plot in director’s way.