Honestly, my observations of real-life body language basically lead me to believe that there are no rock-solid rules of body language!
Everyone is different. At any given instant in time, none of us are exactly the same. Even if we are having a shared experience, we all have uniquely different backgrounds, experiences, and personalities that are affecting every single thing we do.
For example, imagine the body language that goes into someone seeing an old friend they haven't seen in a long time. Imagine all the complexities that will inform the "body language" of that moment! Were they good friends or casual friends? Were they lovers? Did they part on good terms? Did they have a fight? Do they recognize each other immediately or does it take a few seconds? Are they hand-shakers, huggers, or high-fivers? Are they happy or sad to see each other? Or more extreme - are they completely ecstatic or totally mortified to have run into each other?
It truly all comes down to you as the artist envisioning the situation these unique characters find themselves in, letting yourself become an ACTOR, getting into your character's heads and - taking into account everything that makes them who they are - figuring out exactly how they'd react to the situation.
There is no rule that says, "When two old friends meet, they shall look at each other's face for 17 frames, recognize each other, give a 1/2 smile, wave for 20 frames, give a full smile, and then shake hands." Every situation will be radically different, unique, and based entirely on their backgrounds, life story, and personalities.
Sometimes it's nice to have solid rules we can fall back on in order to help us construct a performance. "You cannot lift your left foot off the ground for more than an instant unless you move your butt to be over your right foot." Rules like that are pretty rock-solid, and they sure are comforting when you're trying to work out the complexities of the body mechanics in your scene.
However, when it comes to something as big and all-encompassing as "body language," the only rule I'd say is rock solid is that we are all too different for rules to apply.
But guess what? That's what makes this stuff so fun!
Every performance will be unique. Every reaction to every situation should be based on that particular character's back story, personality, and immediate (and long-term) desires and goals. Because of that, every reaction *should* be unique in its own way.
This specific thing, more than almost anything else, is what will bring your characters to life, and help you craft a performance that is memorable because of its uniqueness and the sense of authenticity that will lend to it.
Have fun, and good luck!