Getting along with an ISTP is deceptively simple, meaning it's so easy you're already doing it wrong by trying to "understand" them. The ISTP, much like the ESTP, has a natural interest in behavior, intentions, and the cause and effect behind how it all interacts together. This raw understanding of interactions, in this case human interactions, is the "simple" part of getting along with them. However, unlike their extroverted brother, when it comes to a baseline need for communication the ISTP is falls somewhere between little to none. The ESTP will proactively engage in verbal and facial expression, while the ISTP observes and engages only if needed. Because the ISTP actively observes what is going on around them, they consider the majority of what most people exchange in conversation to be obvious. The stronger the tendency for the ISTP to observe, the less they will communicate because they tend to take the gathered information for granted, assuming it to be common knowledge.
Upon encountering an ISTP, what is immediately noticed is a vibe of awareness and physical prowess. Along with the fact that they are not expressing much, this is often mistaken as aloofness and most people feel unnecessarily intimidated. On the "inside", however, the ISTP is simply actively observing and more often than not so drawn into what they are experiencing that they can forget their own presence (this is why despite their physical awareness they can be occasionally clumsy). Going back to "deceptively simple", this means that any communication starts at the very least a blank slate and at best in full consideration of the immediate environment or the issue at hand. Although ISTPs are most often skilled which gives them a confident attitude, it is important to separate that from the perceived aloofness so that you can talk to them without feeling unnecessary pressure. The ISTP perceives themselves as being "blank" in expression, and it is upon this blank canvas that many people tend to paint their insecurities onto. Even though something that is obvious to the ISTP may not be obvious to you, that is something they are accustomed to and you should not feel as if you need to know something that you do not. More often than not an ISTP will have strong ability in inductive reasoning which they actually enjoy using for the benefit of others, so don't be shy! If and when the ISTP recognizes you as being more knowledgeable about something, they will simply ask to know more and you should do the same with them. How much and what kind of knowledge one has accumulated is merely a matter of each of our unique life experiences and ought not to be a competition. This unique nature of each of our experiences is why it makes more sense to the ISTP to compete with themselves and remain peaceable with those who also maintain peace.
A common, recurring pitfall in communication with an ISTP is that the combination of perceived aloofness and the expectation to know what seems to be obvious to the ISTP can create a pressure to perform that causes someone's disposition to crumble and impedes communication. Being such a common occurrence, the ISTP becomes annoyed at an inefficiency which they are unable to resolve as well as the assumptions about their attitude that they feel unfairly tread upon their identity which they wish to accurately portray. This annoyance exacerbates and reinforces the other person's initial assumption. This puts the ISTP in a difficult social position where the only comfortable option is avoidance. If an ISTP does not develop a friendly vibe and a better awareness of their own presence they will most likely end up what most other people consider to be a loner. For another person to reverse this tread, they must understand that this is unintended no matter how much it tends to expose the other person's insecurities and to communicate with them in simple terms, with honesty any directness of intention. This is why I say it is deceptively simple, because the ISTP desire for pure observation excludes them from many verbal and facial expressions that make communication so complex. Remember, the assumptions are all in your head, let go of the anxiety and test the boundaries... you might be surprised at how well the ISTP responds.
The last underlined passage is just brilliantly written.