ISTP
Like their fellow SPs, ISTPs are fundamentally Performers (note the capital 'P'
), but as Ts their areas of interest tend to be mechanical rather than artistic like those of ISFPs, and unlike most ESPs they do not present an impression of constant activity. On the contrary, they lie dormant, saving their energy until a project or an adventure worthy of their time comes along--and then they launch themselves at it. The apparently frenzied state that inevitably ensues is actually much more controlled than it appears--ISTPs always seem to know what they're doing when it comes to physical or mechanical obstacles--but the whole chain of events presents a confusing and paradoxical picture to an outsider.
ISTPs are equally difficult to understand in their need for personal space, which in turn has an impact on their relationships with others. They need to be able to "spread out"--both physically and psychologically--which generally implies encroaching to some degree on others, especially if they decide that something of someone else's is going to become their next project.
ISFJ
ISFJs are characterized above all by their desire to serve others, their "need to be needed." In extreme cases, this need is so strong that standard give-and-take relationships are deeply unsatisfying to them; however, most ISFJs find more than enough with which to occupy themselves within the framework of a normal life. (Since ISFJs, like all SJs, are very much bound by the prevailing social conventions, their form of "service" is likely to exclude any elements of moral or political controversy; they specialize in the local, the personal, and the practical.)
ISFJs are often unappreciated, at work, home, and play. Ironically, because they prove over and over that they can be relied on for their loyalty and unstinting, high-quality work, those around them often take them for granted--even take advantage of them.