Let’s look at important things here –
Mercury - Mercury represents communication skills, marketing, calculative ability, younger siblings, hobbies, skills with hands, intelligence, quick decision making, logical thinking, youthfulness etc.
Rohini - It is nakshatra of growing, moving forward and being ambitious. It is a nakshatra of business people. Due to mythological story involved, it is also nakshatra connected with sexuality. For more information on Rohini Nakshatra - https://www.astrosaxena.com/rohnaksh .
Taurus - As Rohini is part of Taurus, Taurus sign and its representations become important here. Taurus is the 2nd sign of zodiac belt, so it represents many things related with 2nd house of horoscope like, Hoarded Wealth, Assets, Savings, Finances, Luxuries and Liquid Money. All in all a sign related with luxuries and all the conveniences of life. Taurus is consisted of 2 and half Nakshatras, i.e. Krittika, Rohini and Mrigshira. Taurus' lord is Venus.
Venus – As Venus rules Taurus, Venus’ position and dignity is important to know about the overall functioning of any planet in Rohini.
Moon - As Rohini is ruled by Moon, Moon's position and dignity will also have an important role in functioning and results of any planet in Rohini.
Houses Involved – Also, we can’t afford to forget the houses involved as a planet can be ruling different houses and sitting in different house for every person as per ascendant and planet’s position. To know overall affect, we have to consider the houses involved as those areas of life are being impacted.
Interpretation of Mercury in Rohini Nakshatra – When Mercury is in Rohini, first of all it is in friendly sign Taurus ruled by Venus. As Rohini is ruled by Moon, at nakshatra level, Mercury is in enemy nakshatra. As Rohini and Taurus both represent beauty, luxury, conveniences and wealth, they can be very much interested in wealth/finance/luxury related things. They can be in business related to these things. As Mercury and Rohini both represent travelling, they can be a traveler by interest or in travelling related work. As Rohini and Mercury both represent Business, being a business person can be the biggest possibility here. Due to flirtatious nature of Mercury and sexual nature of Rohini, they can be highly sexual by nature. They can be lover of beauty. They can gain from siblings but still there can be some frustration or jealousy in relation with them. Likewise, if Mercury is relationship oriented planet for person then same frustration, jealousy or insecurity can come into other relations too. Overall, a very good position for financial gains, beauty and conveniences but there can be issues like person can be too much addicted to pleasures of life, insecure or jealous.
Role of Venus – Venus's position will become important in functioning and results of Mercury in Rohini as Venus rules Taurus.
Role of Moon - Moon's position will become important in functioning and results of Mercury in Rohini as Moon rules Rohini.
Other Factors – And of course, we need to consider conjunction, aspects, house lordship, house position and dashas etc to know the exact results of Mercury in Rohini. So, we always need to take a holistic view of chart rather than just 1 planetary position.
Conclusion – So, this is how I see Mercury in Rohini can work in a chart.
Hope this helps. Please post any comment or query you may have.