Info on Trance/catalepsy

I am an amateur playwright and am wanting to find out as much as I can about certain trance states.

The film 'The Client' with Susan Sarandon and Tommy Lee Jones, features a young child who after witnessing a murder goes into a trance like state. One of my characters is in this state after an attack and I am trying to find out as much medical information as possible.

Can anyone help by giving me some websites or books to read, or have any information/experience of the condition?

Thank you