First impressions count, and in TV and film, even more so; there's no point in hiring an actor to give us a Deadpan Snarker if people think he's seriously being a total ditz until the third act. So when the character comes into the plot you give them an Establishing Character Moment. Other times the Establishing Character Moment may be the small calm when the character carries out something completely unrelated to the plot to show them in their natural element before putting them in an unrelenting storm of plot lines--for instance, during a Morning Routine sequence.