description is easy. all you do is tell exactly what's happening, nothing more.
if you don't mind some advice, it probably comes from trying to explain too much. for an example you might be writting this.
"John gets a drink of water. he is a stern man who has flashbacks from a war long ago. His wife divorced him three years ago and he's resented her for that."
when writing a screenplay you only write what the camera can show, so you basically could leave the last two sentances out. you could write all the ^^^^ but all you would see on the screen is John getting some water, there's no way you would know that he fought in a war and divorced his wife from just watching. (sorry for the horrible example. it was just off the top of my head)
so, i would say dialouge. dialouge is essentially what moves the story along and the wrong words ruin a good scene.