I'd start with a funny walk. Maybe the character leads with his stomach. Maybe he leans back and forth as he walks, as in the sketches above.
Ben Washam said the sitting pelvis should act like a canon ball thrown into the chair, dragging the rest of the body along with it. I imagine the arms would be forward rather than backward as in in this Edward Lear-type pose (above), but this is a funny idea and I'd like to try to try it some time.
Sometimes sitting requires two seating motions: one to sit on the edge of the chair and a second to re-adjust to a more comfortable pose farther back.
Note: none of the drawings or photos here are mine.