For this week we return to a subject discussed before. Perhaps the biggest reason is to emphasize that "the real art of animation comes with the subtlety of timing and spacing".

We have seen a lot of content beyond the balls exercise and I can tell how it affects timing and spacing and how they still walks hand in hand. A quick definition and which affects both:

Timing: 
Is the phrasing of movement. Think like a music. The way you distribute the notes are like you decide to put your key drawings or poses, and it will create the timing.
This influences many other effects:
Weight and Force;
You should remember about what Newton said before: "a body will remain at rest or in motion until acted upon by an outside force";
External and internal force, like momentum. And momentum can't be separated from weight and force;
Don't forget: the property of a moving body that determines the length of time bring it to rest;
Creating contrast in timing clarifies ad accentuates internal state of character.

As you can see, timing remains an important factor that influences what you are animating. And you can't avoid to exaggerate it! At this point you can already notice the difference between styles and how timing is crucial to differentiate them. More you exaggerate, more cartoony you will have and this will be a healthy balance that you will care forever. Try everything!
Some tips to apply before and during your a shot:

1. Know what you are going for before you start it - physics, attitude and personality will ever effect the timing.
2. Put key poses on every frame in order to focus solely on the posing first. "They are more than just mood or dramatics: they also create extremes." It's important to put texture.
3. Start to move poses in time. If you think it need some hold, do it.
4. Play with the timing and see if what feels right.
5. Be aware of the rhythm you are setting up. As a music - again, try to break if is too consistently.
6. You can completely change the feel of a look just changing the timing without change the poses.

Spacing:
Continuing the reasoning and comparison with a music, spacing is how the notes are positioned between the timed keys. In the case, your poses. Will be with your spacing you apply the concepts of acceleration.

The pros agree: don't let Maya make the in-betweens for you, because will be completely unappealing and boring.
I used to track my spacing with software allowing you to draw on top of most applications.

Remember: You can always push poses to create more dynamic spacing or adress spacing issues. If you squash to a pose, you are in essence creating a blur frame, which will allow you to broaden the gaps and create a snappy timing.