Once you have been through the preparing your class exercise, it's a good idea to compare films with books. This exercise draws parallels between printed texts and moving image media and thus strengthens their mutual literacy benefits.
Start with a book the whole class has recently read. Ask them to identify as many things as they can that help tell the story. Everything is useful here:
Write down the answers the children give in a column down one half of the board. Title this column 'Book Story'.
Start a new column called 'Film Story'.
Children may well recognise the additional film factors of actors, sound and moving images. They may come up with additional things like special effects. And they could make some obvious and direct correlations such as dialogue as it appears in print, and the spoken word.
Look more closely at how things like structure, style and mood are used to help tell the story in your chosen book:
Ask the class how these might be portrayed in film. Many of the answers may already be there, they'll just need a bit of teasing out: the action will be fast or slow, it will be dark or light, a character might speak rudely, the music might be suspenseful and so on.
You could use the opening of Harry Potter here - it may be the only book the whole class has read, and if they have all read it and seen the film it could be a useful comparison.