The first question is does anyone even believe in body language? Myself, I put little stock in it, while others swear to is usefulness. Lets take a deeper dive shall we?
Body language believers are those who say the movements of your body is similar to the words you use, except, it is done through the movement of your body, both intentional and unintentional. The most noticeable or easy to read behaviours of body language would include your face - which can make hundreds of decipherable expressions. Add to this your hand and feet gestures, the way your body is positioned during this, and the way you make or point with your hands in the air. A real science of study indeed.
As most would guess, in body language, there is no true language, no fundamentals as is in a true language. Experts agree on one thing, just like economists, that body language can never be truly pinpointed down, its just an estimate guess, and people can have many different interpretations over the same movement. So exact and good is this, and how useful is this in a date? A good way to get body language to have some success interpretation is the use of it with the persons language. That would be a complete indicator for the date.
Facial expressions. This area has to be the easiest to interpret. You can see someone smile, frown, cry, tense, nervous, happy, and the list goes on. That is easy to decipher. Also, even if a person has what we call a poker face, there are usually giveaways as to what lies beneath - tell tale signs. This would be for instance a nervous tick, tapping with their feet, or tapping with their hand, or playing with their hair. Individual people have individual specific traits based on their own unique personality.
For facial expressions, as in all body expressions you are attempting to decipher their feelings or mood. Am I doing good on the date? Does she like? Does she hate me? Are they lying to me when I ask them a question? Are they interested in that person over there and not me? Well, body language can provide you with answers the other person may not want you to know. So, any information you can gleam that can help you on the date is a good thing.
If you want get deeper into the art of body language. The simple act of a smile. That means they are happy pleased. But many researchers have found that a true smile is difficult to do, while a fake smile can be done easily. Also, lets look at the eyes for another indication. It has been observed that people do not have control over their pupils. When the pupil expands, it means the other person is interested in you. Also, researchers noticed that people blink about 20 times a minute (which seems high to me) but goes even higher when someone is interested in you. What researchers have also stated is that the brain processes ones feelings both from the body and the face - and are integrated into ones internal feelings.
Different Interpretations
You may think you know most of them, but depending on ones culture and context, the same movement can mean many different things. For example, the simple movement of the head going up and down. In North America and other western cultures it means a simple yes. In some Asian cultures, it s meant as showing respect for that person. While in India, it lies in somewhere in between where can be interpreted in different emotions.
Body Position
There is a science here, and a fairly accurate one here. Lets look at the most natural body postures with ones emotions. For example, if someone is angry, their body would be in a dominant position, attempting to dominate the other. The opposite is also true, someone who feels scared, would be a more submissive state in their posture.
How Can We Use All Of This On A Date
Well lets look at a few common body movements, and what they can mean for you on a date, success, failure, or a path to enlightenment.
When your date leans in. Well, this is as old as time, and everyone knows this is a good thing, a very good thing. It means the person is interested in you. This is what you want on a date. Some have suggested its a person wanting more of you - again, a very good thing to have on a first date.
What about eye contact? What happens if they are really intent on looking at you, or looking through you with a hard gaze? What does this mean - good or bad? Well, if the person has dilated pupils, as we discussed above it means they are attracted to you. Then again, can you really see someone pupils on a date at night on a dinner date? That one is a bit difficult. Bottom line here, if the person i staring at you intently, for most, it means that person is attracted to you. Always a good thing.
What about their body language. Well, here you can stick with the tried and true principles that we do know work. For example, if their arms and legs are uncrossed, open, it means they are comfortable being around you. If it is the opposite, crossed arms, well, its not going good. If this is the case, the you better open up and calm the individual.
What about mirroring is this true? Some say that if your date mirrors your actions, it means they like you? That one is on the fence for many. Other say that if you mirror the person across from you, they will take that as a sign you like them. Again, even smaller probabilities then their former. As well, a bit too much work.
Conclusion
In the end, stick to the tried and true body and facial movements - happiness, worry, sadness, and anger. They are easy to spot. In the end, go with your gut, enjoy your date, and look for small or large cues that hopefully, the date is going in the right direction.