From this week we will begin to use a new rig, with a torso! And this new element to be animated induce you to discover new elements to care about... The case on this week - and your entire career as an animator - are Weight and Balance!
Whenever something moves, a force was applied to this corps. And what better way to view difficult to the body moves it? Weight. It will help you to sell your intention.
For example, think about a body with a lot of mass, like an elephant. It will need much more time to gain momentum because more mass demands more energy. To stop this mass, you need to think the same: If you have more mass, you will need more time to stop it... And we realize that in the animation should apply the same concept.
"And what I need to know about the balance?"
You need to know balance and weight go hand in hand. The faster the body moves, it will have less balance! You always should apply the concept about the center of mass and put balance.
"Hmm... But what I need to do when I have more than one mass? If my character, for instance, are carrying something?"
Good question! This drawing by Fred Moore helps to understand it. You need to think your character and the the object - or another character - is one body, which means one center of gravity. If he's carrying a box, or other person, doesn't matter. And to show weight, you need to use more curves. If it's heavy, show it! Bend the torso, rotate your head, etc.
"Okay. And if I want my character is intentionally unbalanced?"
You must be careful, because it is difficult to achieve. You should deliberately miss the center of gravity and be more natural as possible.