Before going into screenplay writing, I just want to throw some light on what writing by it self and how screenplay writing is different from other forms of writing.
Why do we write ? the fundamental reason is to express oneself.
It can be in any form from a essay of 200 words to a novel of 2000 pages.
The reason might be all same but the purpose of writing might differ. Like, some one writes a novel, the purpose is to read it, similarly some one might a poem so on.
When some one writes a novel or poem, a third person reads it, there is a lot of room to imagine things. The challenge might be, how close is the writer thought and the readers understanding, or some times the writer might want the reader to take things in his own way. In this case when a writer writes some thing as a novel or a poem and a reader reads, the purpose is complete.
But in case of screenplay it's not so, It's the first step in the production, or the first step towards the purpose. Then every one involved understands it and produce a film. So here in case of screenplay the gap between the writers thought and the readers understanding should be as minimal as possible, that's the biggest challenge.