ISTJs are so consistent and can be easily frustrated by the inconsistencies of others, especially when these others don't adhere to their commitments or agreements. They do, however, usually keep their feelings to themselves unless they are asked - but when asked, they are unlikely to mince words - facts, truth and telling it like it is will always win over tactfulness. Serious-minded, individualistic and thorough, the ISTJ may focus so much on the task that they forget the needs of others - including themselves as their focus can be laser like and intense. Others may see them as slightly cold and impersonal, possibly even as uncaring. This is merely focus but can alienate those trying to get close.
As their decisions are based on what makes the most logical sense, this can mean the feelings of others may not necessarily be factored into this process. ISTJs often give the initial impression of being aloof and perhaps somewhat cold. It is possible for the ISTJ to express emotional warmth but not without depleting their energy resources and so they will then have to move out of the spotlight and spend time alone drawing energy from within. They are factual and need time alone and the more effusive extravert types may find that a little difficult and, at times, disconcerting.
Detached and factual ISTJs often find it difficult to deal with emotions as they see these as irrational and illogical, and when others display emotions they have to translate the emotion into factual language that they can understand. Deep and private ISTJs will tend to keep their own feelings to themselves and until they have the measure of people will be unlikely to proactively share their feelings. Too much interaction sucks their energies. ISTJs like people to stick to agreements and commitments and may not understand that this is human nature, not human deficiency as they themselves take such commitments incredibly seriously.
ISTJs prefer to keep themselves to themselves and so in conflict situations they will struggle with overt conflict, ie with someone who wants to engage verbally. They are fine with more formal conflict. ISTJs have a strong sense of right and wrong and this tends to be around what has been agreed. They believe if you have said you’ll do something then it is akin to a contract and you should fulfil your obligations. Formal processes such as writing letters of complaint or going through the proper channels plays to ISTJ strengths. They are measured and formal and so will be very factual and non-emotional. It is all about right and wrong. ISTJs do not like conflict if it involves emotion, as they prefer to see things far more factually and in a binary way. For them it will be about did it play out fairly and as agreed.