Complex sentence: Script editors of the new series do not have the power to commission scripts, instead, they act as liaisons between the production staff and the screenwriter, before passing their joint work to the Head Writer for a "final polish".
Split into simpler sentences: Script editors of the new series do not have the power to commission scripts. Instead, they act as liaisons between the production staff and the screenwriter, before passing their joint work to the Head Writer for a "final polish".