It's introversion plus Te that gives the robotic exterior for some ISTJ (more so the males, IMO). Te-dom are usually much more animated because they are extroverts & they have an influence of tertiary Pe light-heartedness, but Ne is inferior for ISTJs (can lack playfulness). To top it off, whatever feeling they have developed is introverted, so you don't see much of it unless close to them (but they can have very deep feelings).
Si is kind of sentimental though. ISTJs are kind of crusty on the outside, smooshy on the inside, IMO.