Mercury in Sagittarius:
Freedom of thought is what Mercury in Sagittarius strives for — at least in theory. These people communicate in an optimistic, forward-looking manner. Their ideas are generally very big, but it’s probably wise to use tact when you see all the little holes in their arguments. Remember, they mean well. These people have vision.
Not always thrilled about focusing on details, Mercury in Sagittarius people prefer to expand their mental horizons through thinking and reading about larger subjects, like philosophy. Their thirst for knowledge can be insatiable, but they have little patience with dry academics or cold, hard facts. Theirs is a restless intellect. They learn best when they are given the freedom to think for themselves, when they are truly interested in the topic at hand, and when the things they’re learning are fun.
Even though their definition of justice might bewilder some, Mercury in Sagittarius defends justice and freedom religiously. They generally have a whole slew of principles they follow, or at least defend in conversation, and they’ll gladly discuss them all with you.
Their optimism is both their strength and their weakness. After a conversation with these people, you’ll probably feel stimulated and invigorated. They make life a little better for their positive, optimistic outlook. But sometimes that optimism can be a little blind to reality.
Organization, too, is not a forte of Mercury in Sagittarius. Who has the time for organizing anyway? When the clutter around them gets too unbearable, they’re given to trashing or piling things instead of putting them in their rightful places. They do appreciate lots of room, or at least the illusion of space, when they’re working, studying, or thinking. So, their office might look alright at first glance. Open up a drawer or a closet, however, and you might find it’s bursting its seams.
Conversations with Mercury in Sagittarius are always interesting. You can’t help but admire their enthusiasm. At their worst, they take to preaching instead of conversing. This may not be intentional, and can get Mercury in Sagittarius into all kinds of trouble nonetheless. A good debate is hard for people with this position to pass up. Mercury in Sagittarius natives think they are very open-minded–and they are in spots–but they can come across as quite moralistic at times too. They’ll be the first to spot bias or prejudice in everyone else’s communication, but they can be a little blind to their own. They’re not always the best listeners, either, and not adept at picking up the subtleties in your conversation.
Bluntness of speech is a trademark of Mercury in Sagittarius. These people get impatient with indirectness and beating around the bush. It baffles them. It may even piss them off for a while. A sure-fire way to get under their skin is to play mind games with them. They may not always pick up on the game-playing at first, but when they do, they won’t be amused.
When they’re not fired up about some issue or another, these people are fun-loving. They love to laugh, and it’s usually pretty easy to get them giggling. (Source).