I was looking through you sketchbook again and something that could help you "unstiffen" you drawings is to do quick 1 or 2 minute gesture drawings. Since by the the things that you posted lately are full of detail you seem like a rather neat person so try to loosen up with gesture drawings and fast sketches. You can also try to exaggerate the actions that you characters do even though it might look unnatural. It could help you find the right balance.