How the thinking works
Concentration is the defining feature. This Mercury goes down rather than across, staying with a single question long past the point where most minds have declared it settled, and the depth it reaches is genuinely difficult to match.
Motive is assessed before content. Told a fact, this placement first asks why it was told, which makes it excellent at detecting evasion and prone to finding an intention behind something that had none.
How it speaks and writes
Speech is economical and controlled. This Mercury asks a great deal more than it answers, and it will let a silence run rather than fill it with something it does not want on the record.
Writing is dense and unsentimental, strongest when the subject is something concealed. Its natural mode is investigation, and it loses interest quickly when there is nothing left to uncover.
Where it goes wrong
Suspicion generalises. A pattern learned from one deception is applied to people who are simply being brief, and this Mercury spends effort on threats that were never present.
The other cost is the asymmetry of disclosure. Questions go one way for years, and it is usually the other person who eventually notices, which registers to them as a kind of withholding even when nothing was being hidden on purpose.
Why the dignity is what it is
Mercury is peregrine in Scorpio. Mars rules the sign traditionally, with Pluto given the rulership in modern practice, and Mercury holds no dignity or debility in either reading.
Without its own ground, Mercury adopts the sign's method: fixed water concentrates and does not release, so information is gathered intensively and retained rather than circulated. Peregrine is a description of standing, not of strength, and dignity throughout describes fit between planet and sign rather than the quality of the person.
Against the opposite sign
Mercury in Scorpio faces Mercury in Taurus, which accepts the surface when the surface has held up over time. Scorpio cannot take a plain account at face value; Taurus cannot see why anyone would spend a month checking one.
The axis is the useful unit. Scorpio's correction is the Taurean willingness to leave a settled thing alone, and it tends to be learned after the investigation has cost more than the finding was worth.
Questions people ask about this
Is Mercury in Scorpio suspicious?
It reads motive before content, which is accurate often enough to be reinforced and wrong often enough to be a real liability.
Why does this placement give so little away?
Disclosed information cannot be withdrawn. The reticence is a calculation rather than secrecy for its own sake.
Why is Mercury peregrine in Scorpio?
Dignity names fit, not grade. Mercury has no traditional dignity in Scorpio under either the Mars or the Pluto rulership, so it works on the sign's terms.