You can change which Characters appear in a scene directly from the event node in your nodemap.
Changing a Character in a scene
Open your Story and go to the nodemap. Find the event node that controls the scene where you want to change the Character, then click Edit.
In the event node, select the Character you want to change. You can swap them for another Character from your cast or remove them from the scene.
Once you've made the change, continue through your Story and check any other event nodes where you want the new Character to appear.
Event nodes set which Characters are present for the dialogue nodes that follow them, so changing one event node only affects that part of your Story.