What is a screenwriters role in making movies?

Answer 1

Your answer depends on both the screenwriter and the producer.

It is possible that a screenwriter was paid in full for the script, in which case another screenwriter may be hired 'on set' to re-write scenes depending on variables that dictate edits. For example, if the original script was set in Chicago, and the shooting location is Monte Carlo, certain elements of dialog or plot could be adjusted to accommodate this change.

There are movie sets where certain actors 'demand' more lines, and the producer and director agree. In these cases, the screenwriter adds dialog -- screen time -- for the actor making the demand.

Finally, there are 'script doctors' who re-cast completed scripts according to 'notes' given by other participants, such as producers, director(s), actors and so forth, prior to finalizing the shooting script.

Answer 2

Every movie starts with a script, and it is the screenwriter's role to write it -- in the beginning. Some screenwriters also work on subsequent drafts of scripts -- re-writing them, and there are screenwriters who work on shooting sets, to craft scenes or re-craft scenes to fit the vagaries and demands of on-set work.