On the other hand, if you create characters without basing them on real behaviors or things you've experienced about life and human behavior first hand, you invariably end up basing them on characters that are just obvious clichés or copying characters that have already been created, and they will end up feeling flat, lifeless and dull.
Understanding what makes people behave the way they do is a big subject, obviously. There's no replacement for studying the people around you and observing people "in the wild" whenever you can. You should always try and analyze what people are doing and why. This can really open your mind to new ways of thinking about how people think and how their mental state shows through in their expressions, gestures, body language and speech.
There are, of course, many books on human behavior. I definitely recommend spending some time reading about and researching this area. It can really help you make sense of what you're seeing when you're people watching, and it can lead to ideas for acting and behavior that you wouldn't have thought of otherwise. Here are a few books that I've enjoyed on the topic (although I read all of them years ago, so I can't give you too much in-depth information about them):
The most comprehensive book on the subject of human gestures (that I know of) is Desmond Morris's Manwatching. Morris has written several books on human behavior, as well as ones dedicated to dog, cat and equine behavior. Manwatching was published in 1977 and is out of print right now, but if you are interested in a copy, try alibris.com or abebooks.com. It looks like you can pick up a copy staring at under $2.00. As a note of caution: it appears there is an abridged version called The Pocket Guide to Manwatching. I don't know how much shorter it is…I've never seen it. I'd recommend getting the full version of the book.
This is a book about actions, about how actions became gestures and about how gestures transmit messages. Psychologists have long studied what makes people tick. Now Desmond Morris reveals what makes people twitch, stare, grimace, point, poke and shrug. Here is a complete and fascinating catalog of human behavior, closely examined in Morris's lively text and hundreds of telling photographs, drawings, and historical prints. In this captivating anthology of body language are the postures, hand gestures, and facial expressions that accompany our true feelings, often hidden under the mask of convention. Here is how we pantomime the meaning beneath our outward behavior in the whole range of social situations.
I photographed and transcribed a few examples from the book, just to give you an idea (click to read the caption of the photo below):
The text referencing this picture reads: when a man and a woman have to squeeze past each other, the man twists towards the woman, while she twists away from him.
Manwatching is full of things like this: seemingly small pieces of observation that you never thought about before. But after reading the book, you'll suddenly start to see that these things happen around us all the time---without the participants even realized they're exhibiting any kind of learned behavior!
And the text for this picture reads: the Body-cross-a temporary barrier formed at moments of tension. When individuals feel exposed or threatened they often form a barrier across the front of the body by making contact with themselves. This may be done with a simple arm fold, or may be disguised as a small grooming action, such as (below) a cufflink adjustment, handbag attention or bracelet checking. This need is felt even by experienced public figures, especially as they cross a threshold on a formal occasion.
Manwatching is truly an exhaustive study. Morris does a good job of tracing the history and evolution of many gestures, and he goes to great lengths to describe the differences in gestures from culture to culture. What's considered perfectly acceptable in one culture may be horribly offensive in another, and many gestures with deep meaning in one culture are completely meaningless in another.
I definitely recommend Manwatching, but I should warn you that it does read like a bit of a textbook, and the overwhelming amount of information may make it seem like a daunting read to some. If you want a simpler, lighter version (or if you, like me, are fascinated by the topic and want two different sources on the subject), you might take a look at The Definitive Book of Body Language by Allan and Barbara Pease. It looks like the hardcover is still in print and available. But it's been around for a while, so I'd bet you can pick up a used copy for less, if you're interested.
This book is definitely easier to read and more digestible than Morris's book, but (if I remember correctly) isn't as in-depth as Manwatching and doesn't devote as much time explaining how the gestures evolved over time. Here are a couple of examples to give you a feeling for the book:
An explanation of what the gesture means when someone stands, holding their hands clasped behind their back:
The book seems well-researched and has some great insights. One strange side note: it is full of very simply drawn artwork to illustrate certain gestures (as opposed to Manwatching, which is prodigiously illustrated with photos as examples, but also has some detailed drawings for reference). I'm surprised the authors of The Definitive Guide… didn't seek out a better illustrator. But the illustrations serve their purpose, and that's all they need to do, I suppose.
Here's one of my favorite bits: an explanation of why people act so strangely on elevators (it's because we are not used to people being in our personal space, but that's a necessity of being on an elevator, and so it causes us to adopt weird behaviors for the duration of the ride):
I hope you find these recommendations helpful. Obviously, none of these books is a "Bible" that must be followed absolutely, and they should not be treated as formulas for acting or emoting. The best characters are always based on the thoughts and ideas of the people that created them, and not based on a catalog of human behaviors. But all of these books can be helpful at setting context of what certain gestures mean and helping to understand why we act the way we do, as well as giving insight into what a person is thinking and feeling when they move or act in a certain way. I've definitely found all there of these books to be interesting and inspiring and I hope you will too.