Don't be fooled by the word "Supporting". After all, a building without supports is bound to collapse. Anyway, script editors are essentially a second line of writers who edit the scripts. They are expected to be of the same level of skill as writers are expected to have. Mastery of Grammar and Spelling is key, and to edit something like, say, dialogue, one must know how to WRITE dialogue. Keep in mind, certain episodes will already have been written by the time you are selected, however you as well as the other Writers and Editors may edit and re-write them, with permission and under supervision. If Writers are the spinal cord, Editors are the vertebrae.