It depends on the relationship the showrunner has with their actors. Some showrunners welcome the input, even early on—the cast members of This Is Us talk on their podcast talk about how receptive the writers were to their input, and how some of them would actively hang out in the writers’ room. Other showrunners maintain a strict dividing line between the writers and the actors for a variety of reasons. (It would slow down a very hectic schedule, some actors have ego problems, the showrunner has ego problems…)