Here we have our structuralists, the people who make the bumpers at the bowling alley. Mercury in Capricorn knows how to create a path that others can follow, pays attention to every important detail, has organizational acumen, likes things to be orderly, and puts things in their rightful place as a hobby.
Mercury in Capricorn are excellent disciplinarians. They can come off rather cold or dry, but they are rather focused on delivering highest possible fidelity of information, using the correct words in the correct order. So their sentences can sound very well-crafted, and they can get their point across very well. So we like to see these people in key communicative positions, writing out standard operating procedures, synchronizing calendars, taking meeting notes, writing code, making plans, or helping others figure out exactly how to get from point A to point Z. As such, Mercury in Capricorn can make excellent engineers (software, civil, mechanical, etc.), physicians, writers, coaches, physicists, construction workers, architects, business owners or managers, accountants, executive assistants… anything or anyone that makes things function well and sustainably. Additionally, chiropractors, massage therapists, nutritionists, Ayurvedic doctors, herbalists, any kind of healer that uses the structure of the body and basic science of health in their work or as a hobby can.
You might not get tons of excitement from Capricorn Mercury, and they can talk to you without the typical babying and sweetness of Mercury in other signs. Rather, you are talking with a real adult, who does not always take themselves seriously, but who knows when it's time to work and when it's time to play. And their play could also be rather educational or formative. Kids might actually love being around this kind of person because the boundaries are clear and they know what they are getting. Plus, the play here teaches real life skills.
Mercury in Capricorn could be the kind of person that builds websites, writes dictionaries, creates business plans, helps organizations scale, assists small businesses in building out systems, teaches language/grammar, writes out step-by-step instructions, creates recipes with exact measurements (and thinks of possible substitutions), makes great use of a planner, keeps everything important on their calendar, makes lists before they go out grocery shopping, writes out exact expectations when disciplining their children or hiring a service, and repeats the same things one thousand times with patience until it clicks.
Emphasis here is not on moving quickly or having the most fun ever, rather, it is on sustainability, sound foundations, real understanding, and the possibility to replicate information and pass it along to the next person without losing quality of information.
Mercury in Capricorn can therefore enjoy spending time with elderly or orderly people, or just find themselves in the company of more grandmas and grandpas, through work or circumstance. They may have had extensive contact with various elderly people when they were growing up, and therefore use some of their more outdated phrases, or take on some of their more conservative qualities/values. For instance, Mercury in Capricorn could be a bit more tight with their purse, choosing to place their money in something very reliable like treasuries or SPY rather than speculative assets they're not too sure about.
Mercury in Capricorn can also be a naturally talented craftsperson working with their hands. They can work with metals or stone or other kind of Earth materials. They might love to have plants at home, for example. Or an entire garden that they love to manage on their own. They could just have a companion pet, which they might train rather than leave them untrained. They can build things with their hands, like making furniture, or even refurbish old furniture by giving it a facelift, so they might really love to have some kind of creative project that lets their hands touch something Earthy. Similarly they can work in landscaping (or enjoy taking good care of their yard), have extensive knowledge about herbs or different plants, and have an at-home apothecary where they make use of old, natural remedies. They might emphasize being self-sufficient, and could enjoy doing things like canning their own goods, or challenge themselves to buy less from the grocery, or make much less waste somehow. Similarly, they are likely to enjoy doing various projects around the house, fixing things up here and there, and they may keep old things so long as they are continue working. Some extra time and attention can be spent on maintaining things. And this maintenance can also be maintenance of body-mind-spirit. Some Mercury in Capricorn can have a trusty routine they don't go without – frequent gym, mandatory therapy, going to the salon for quality hair treatments, taking supplements for their skeletal/organ health, preparing meals in a particular manner to avoid health flare-ups etc…
Growing up, they may also experience being very much corrected in the way that they speak, and rather than receiving cuddles and coddles, they could experience having more cold, distant familial relationships that are focused more on being productive, accomplished, proper, well-behaved. They can frequently feel they are being unfairly criticized, and overly disciplined. They can have games taken away from them, for example, and instead be asked to do loads of homework or chores, or even to take on more adult responsibilities and raise their little siblings or help their parents in meaningful ways, for example by doing unpaid labor from a young age. Mercury in Capricorn can also be somehow passionate about the environment or about the Earth, very much enjoy the energy of trees, forests, mountains, anything that is real and natural, and they may choose to surround themselves with such elements by keeping plants, various stones, grasses, metals… anything from the natural world that can give their home a grounding touch. Lastly, Mercury in Capricorn is likely to investigate things that are old and historic – like old buildings, castles, historical periods, antiques, carbon dating of rocks or trees, ancient peoples and cultures, or even pre-historic conditions… they are interested in the past and how it shapes the present.