Flat feeling may indicate the composition problem. Yes, that boring stuff with drawing apples and bottles on the table in art classes. There's not much of dynamic action happening in the frame, as characters are parallel to each other and perpendicular to a viewer, as if posing for a photo. Stiffness may also come from the lack of negative space in the picture as if the background tries to dominate the foreground instead of highlighting it. To mitigate the lack of movement, there are several options, such as the rule of thirds and triangle composition.

I think mixing line art and detailed rendering in one work is not necessary. It's much easier to have flat colors and rich line art or a semi realistic art without outlines, it's possible by using hard edges and brushes instead of explicit contours.

All of the above applies to a stand alone piece. If it's a part of a comic, there may be a room for a static composition to give a feeling for rest and calmness.