In the past, I've recommended "In the Blink of an Eye" By Walter Murch, which was recommended to me by a few animators back in the day and continues to have great insights into filmmaking and behaviors every time I open it up. There are other great acting books and so forth, but honestly, I think the best advice is to just get out there and crank up your observation skills!
Go to a park or a beach or a cafe - somewhere that you can observe people interacting or running around -- and just really take a look at the people around you. What are they doing? How are they moving? Why? What happens when she reaches out and touches his hand? How does the little kid blink when he sees the ice cream cone? When the dog is chasing after the ball, how are his hips pushing him along, and how does that timing relate to the way his shoulders react?
There is life happening around us EVERY DAY. All we need to do is pay attention. If you see something interesting, write it down or do a little sketch to remind you of a pose or action. Keep a notebook in your back pocket or work on having a strong memory for actions and emotions so that you can draw on these inspirations and observations when the time comes to use them in your work.
If you're talking about a specific action or emotion that you need to study for a specific shot, then my advice is to seek out reference anywhere you can find it, including in the mirror. Check YouTube, your dvd library, the BBC motion library, your TV, favorite movies, your family, friends, neighbors - it doesn't matter where you find the appropriate reference. What matters is that you find it. And what matters even more than that is that you THEN take the time to make a careful STUDY of it. Don't just look at it! Observation is not just "seeing" something. It's STUDYING it. Figure out the WHYs of it.
Noticing what is moving in the body is great, but it's far better to figure out WHY. That way when you're deep in your scene, you'll know exactly what to do, because you'll know WHY the timing and poses have to be the way they have to be.
Best of luck!