Mercury in Scorpio:
These people are on an eternal quest to get to the very heart of any matter. This appears in anything that requires thought, and in almost every conversation they have. Totally fearless when it comes to delving into depths that nobody else wants, or even thinks, to explore, Mercury in Scorpio is adept at learning the source and the core of any problem or issue.
They are blessed (or cursed, depending on how you want to look at it) with the observation skills of a surveillance camera, seeming to just see all and know all. Sometimes they can be quite “dark” because of their tendency to focus on the false in order to get to the truth. These people detest superficiality. They come across as suspicious and can annoy other, more optimistic people with their negative take on things.
Scorpio communicators are passionate in their speech, sometimes even given to lecturing. Remember, though, that they are just as passionate about giving you real advice and help. They are quite willing to let you in on a few of their secrets–regarding other people, that is, not themselves. The truth is, they have observed things that many have not. If their observations seem a little bit skewed toward the negative, try to overlook it.
You may just discover that they’ve given you some of the best advice around. They are at their best when they are talking about things that they are not personally involved with. That is when their shrewd judgment shines. It is when they are personally involved that their judgment can get clouded over.
Their intelligence is absolutely instinctive. Strategies are their specialty. Their idea of constructive criticism can be interpreted as destructive criticism, depending on their audience. It can be very hard to win an argument or debate with Mercury in Scorpio. They are absolutely driven to win, not just for the sake of scoring intellectual points, but simply to come out on top in general. Besides that, it is hard to deny that their thinking is really quite lucid, at least in presentation. You may be gritting your teeth, but it will be hard not to concede.
They’ll staunchly support and defend anyone they care about. And, their energy in speech (or sheer intensity) can be motivating. You may walk away from a Mercury in Scorpio native, after they’ve given an impassioned speech, saying “whew!” But you’ll generally have received an awesome pep talk, if they like you. The pep talk may have been at someone else’s expense, however (they may have torn someone else apart to boost you up, for example).
Scorpio’s take on people often has to do with motivations. Often, they feel people are motivated by jealousy. They should know, if they are also Scorpio Sun and/or Venus Scorpio, because they have lived it.
They are naturally suspicious, expecting bad intentions first. Mercury in Scorpio people love intrigue, mystery, and hidden motives. If they don’t channel this interest into mental pursuits, they can end up creating their own mysteries. Others might start wondering if they are paranoid.
Mercury in Scorpio loves anything that challenges the brain and are often interested in what makes people tick. (Source).