Social engineering is defined by Wikipedia as “the act of manipulating people into performing actions or divulging confidential information, rather than by breaking in or using technical cracking techniques; essentially a fancier, more technical way of lying. While similar to a confidence trick or simple fraud, the term typically applies to trickery or deception for the purpose of information gathering, fraud, or computer system access; in most cases the attacker never comes face-to-face with the victim.”
Over the years I’ve done my fair share of social engineering, and the one thing that I have always found to come in handy is being able to read people’s body language. Being able to notice when someone is pacifying themselves, when you ask certain questions, and knowing where to hone in on for example, has helped me countless times in the past. Being able to notice the little things like when people are extremely nervous when you mention things like “Well, I’m not too sure Mr Jones, you manager would be too happy about me not being able to gain access to this room, as he’s paying me to have a look around in your data hall.” When they’re blatantly telling you, that they can’t allow you access under company policy, etc, etc.
I would encourage anyone that performs penetration testing that includes social engineering exercises, to really take the time to read up on body language and how you can make it work for you, it will help your social engineering skills, and this will also help you to help your clients.
There are countless books on this topic that you can get from most decent bookstores to help you along your way, and the good news is that some of these are really not expensive at all.
Another thing that you may want to look into is reading micro expressions, although I would recommend that you start with learning basic body language first, and then progressing on to micro expressions.