It's really flat and really stiff.
Give it some movement:
Try thinking in curves as you draw. Everything curves. No straight lines. Take the center line through everything and make sure it curves. This is your gesture. You want it loose and comfortable.
Think "opposition". If something on the right is forward, then something on the left goes back. One side is up, the other is down. The artsy term is contrapposto, which means "counterpose". The body keeps balance with itself this way. Keep in mind that
we're built for it when you draw people, no matter how cartoony.
1. Get up and stand with your weight evenly in the middle of your feet. (Feet not touching. Just stand naturally centered.)
2. Without holding onto anything, lift one leg. How did your hips move to counteract?
Always consider this when you're drawing people. The angles will make things move dynamic and proper balance makes things more natural and believable. Join the opposition with the curves and you get appealing movements and shapes that create clear gestures.
Give it some substance:
Draw in 3D. Always remember that. The most simple rules to make sure you're always drawing in 3D is to make sure everything has a
top, bottom, and sides (these are called planes) and
show space things take up on the ground.
Give it shape:
Look at it an pretend it's all filled in black. That's the silhouette and that's the first thing people see. If there is no silhouette or it's really boring, the image isn't very appealing. This comes back to opposition. Having the opposition keeps your silhouette from being symmetrical.
Even if someone is just standing there, they're still DOING something. Take this for example. Check out the picture at the bottom and the silhouettes taken from it in the middle left of the critique page. They're all just standing/sitting around doing nothing, but you can still clearly see that their bodies do something. They lean. They rest. They support themselves. Start to take notice of what your body does when you're doing nothing. It helps with the drawing. Trust me on that.
Shape, movement, and substance will bring it to life. This is a pretty quick overview, but if you remember and check that you're doing these few things each time you're drawing, you'll see immediate and dramatic improvement.