A standard way to make someone like you is to mimic their face and body motions. Smile, cross your legs, etc. when they do. Why does this work? Some say it helps you empathize with them, to feel as they feel. Which is almost right. But more precisely mimicry helps you to see them as they want to be seen, rather than as they really are. From a recent Psychological Science:
Mimicry facilitates the ability to understand what other people are feeling. The present research investigated whether this is also true when the expressions that are being mimicked do not reflect the other person’s true emotions. In interactions, targets either lied or told the truth [about donating to a charity], while observers mimicked or did not mimic the targets’ facial and behavioral movements. Detection of deception was measured directly by observers’ judgments of the extent to which they thought the targets were telling the truth and indirectly by observers’ assessment of targets’ emotions. The results demonstrated that nonmimickers were more accurate than mimickers in their estimations of targets’ truthfulness and of targets’ experienced emotions. The results contradict the view that mimicry facilitates the understanding of people’s felt emotions. In the case of deceptive messages, mimicry hinders this emotional understanding.
You are attracted to those who mimic you in part because such people show you that they will try to believe your lies. You like people who will uncritically accept your story. Do you want to reconsider if you should mimic others, or like those who mimic you?
It seems true that this works in general, but simply mimicking body language is not the only thing happening here, when it does work. For example if there were a person waiting in a queue at a bus stop and you started mimicking them, it would all be a bit weird. It certainly wouldn't build trust.
Things are happening on more levels than just pure body language. I think you'll find the body language is secondary to the main body of communication. For example, nodding at each other whilst saying how bad the service is at the local supermarket, the conversation being profoundly negative, however strong positive messages are causing bonds to be formed. However, have you ever had the case where you go along with things, agreeing, nodding, bemoaning, only to walk away relieved that the ordeal is over. It is amazing how easily we mimic, we can slip in and out of this behaviour without noticing. Interestingly these behaviours are generally lacking high functioning autistic spectrum people. One interesting question, are mild autistic people perceived as socially challenged because they lack these traits or do they lack these traits because of being socially challenged. Unfortunately for high functioning autistics the answer doesn't help them, only the people who occasionally get a dose of truth from them.
Another point; if you were to nod in agreement, mimic and so on, but never make eye contact, or worse, distinctly look to one side, or up into the middle of your fellows forehead, the mimickery effect would be destroyed. All messages have to be congruent and we are acutely aware when they're not. People refer to this as instinct. Being nervous can thwart your normal behaviour and send mixed messages. Also most video conference facilities for people to look at a screen, whilst the camera is elsewhere, can actually cause distrust (search for the MIT paper on this subject). I would argue that eye contact is probably more important than mimicking.
An interesting study I'd like to do one day to collect empirical evidence (excuse to go out drinking) is how body language can communicate across a larger distance. Identify people in a bar that are perhaps interested in each other, or conversely, perhaps competing with each other. They're body language synch's up, despite being quite far apart and perhaps not consciously aware of each other. Taking this a step further you can actually mimic someone across a room and the results can be quite freaky if you're the sort of person that likes to play such games, especially when alcohol is involved. Other examples are where groups identify with each other through posture, gesture and how they adorn themselves. This can be spotted from great distances and is fundamental to society.
Mimicking someone to make him or her like you is like sucking up to someone to make him or her like you. Only subordinates do it.
Usually, you won't be able to change your status by consciously choosing to mimic or not. But, if you could, you would rarely want to signal your willingness to be the follower if you aren't already.