Select the character model, enter Edit mode, and in Side view, press Ctrl
key and with a left mouse click ( on the front area) start to draw the ouline
face line ( always pressing Ctrl key).
In Front view , the face outline line of vertices must look like a straight
line. With the box selection tool, B key, select all the vertices in the
line, and move them to the middle of the mouth. Select the vertices that
forms the mouth's edges ( in Side view) and Extrude them, three times in
Front view.
In Side view, move the vertices to fit the mouth's shape.
Control your work with Open Gl shaded mode.
Wireframe view of the face outline and his "automatic double".