This placement can yield individuals that are particularly interested in the world of symbols. For instance, Mercury in Sagittarius could have extraordinary dreams, read cards, have psychic impressions, astral travel, practice reiki, perform witchcraft, or anything thereof. Inches more towards the weird and inexplicable, which can also be reflected in their art style, by creating things that are abstract or angelic/demonic or just unfamiliar to most people. They can certainly be attracted to more conspiratorial, spiritual, or philosophical types of conversations, where they tend to explore various concepts that are not necessarily rooted in facts. For example, they might study different religions, have conversations about the afterlife, take interest in aliens, explore the vastness of outer space, watch videos on various conspiracies, etc…
Mercury in Sagittarius can also be rather loud people in different ways. For instance, they could have tattoos, piercings, or a fashion style that immediately gets attention. Or they might have a loud voice, or perhaps speak with a thick accent, listen to loud music, or produce various sounds, sometimes speaking in unrecognizable tones. Mercury in Sagittarius can also be in positions where they have to shout, or speak loudly, or over a megaphone, or a speaker system, anything that amplifies their voice, even doing loud whistling sounds or partaking in screaming matches… oh, they could certainly be in loud environments where there is loud music playing, like DJing or just enjoying going to the clubs, or there might simply be loud people in their environment, like loud neighbors, or parents that are having yelling matches, etc.
They could also make up their own languages or codes, and therefore be intrigued by morse code or sign language or foreign languages. And they might speak foreign languages regularly, or otherwise frequently interact with people who live far far away or do not speak the same language. They might like to use whatever words they know in a foreign language even if it doesn't come out quite right, or just enjoy meeting foreigners and practicing language skills with them.
Mercury in Sagittarius can also be very funny and enjoy comedy. More easygoing in terms of attitude, more open to different types of people and conversations, more open-minded in every way (unless there are contradicting aspects), and very quick to come up with new ideas. They are actually fascinated by the world we live in and love to know more about it. So they can absolutely be into the Youtube rabbit holes, watching various moves and documentaries, listening carefully to their favorite artist's lyrics, or reading lots and lots, just absorbing different kinds of information, and their interests can frequently change, or they may have several careers that are each quite different from each other.
Sagittarius Mercury can also be rather religious or spiritual. So they can speak in parables, quote various holy texts, listen to spiritual teachings, notice angel numbers, explore different systems of higher knowledge like Astrology, human design, numerology, cardology, etc…
But there are many Mercury in Sagittarius who are also strictly scientific, who, rather than seeing the world from a spiritual point of view, describe it in various philosophical terms, but still take interest in attempting to explain the whole of existence and "God".
While they can have good humor, some Mercury in Sagittarius can be very intense, demanding, taking the high ground, forcing their opinion onto others, not listening, interrupting, or have very strong philosophical disagreements with the people around them.
They may also get particularly lifted into various forms of gambling or addiction. Given the Jupiterian influence, they could partake in various psychedelic experiences, like taking mushrooms or LSD or ayahuasca, or even have prophetic or lucid dreams, or frequently daydream but to a much greater extent than the normal person, so they can really get carried away and lose focus. Similarly, they might like the idea of getting big fast, so they can have some ridiculous ideas/projects about how to make tons of money quickly, like day trading or investing in crypto or buying up assets they think will eventually explode in value, and can similarly love to play video games where this rush can be equally simulated.
Mercury in Sagittarius can be optimistic, high-minded people, they don't focus on the negative, they try to bring up the energy wherever they go, lift the mood of the people around, or just make the environment more fun and convivial. Once again, they could accidentally dominate in conversations without catching themselves going on and on, but sometimes this works very well, especially for those who love to be on stage, or who are in positions where they need to talk a lot and for long periods of time, for example being a lecturer, a politician, a representative, a lawyer, a judge, etc.
Mercury in Sagittarius asks loads of questions, joyfully participates in most activities, and takes people on a journey to somewhere they have not been before mentally. This is what makes Sagittarius Mercury extremely funny, intelligent, learned… and sometimes arrogant, overconfident, and too loud.