The relationship between the screenwriter and the filming process is a complicated one. Every production is different, and every writer is different. Some productions don't want the writer around, and some writers don't want to be around the production. Some writers are so keen to be involved that they take the drastic step of becoming directors (I'm looking at you, Graham Linehan).
A writer's status in a production can range from extremely low (a first-timer writing a sketch for an established star) to extremely high (Steven Moffat showrunning Doctor Who). My writing partner Jesse Armstrong and I have run the gamut. On one of the first sitcoms we were hired to write for, we were keen