As with all interpretation, it’s important to look at the natal chart. Maybe the person has a natal Mercury – Neptune so when a Neptune – Mercury takes place, the person wouldn’t be too foreign to the energies as they have been living it throughout their lives. Let’s say Mercury conjuncts Saturn and is in a fixed sign. When transiting Neptune hits Mercury, you can bet that the person may not go through a particularly enjoyable time, mentally speaking of course.
Let’s say, we have a person with Mercury in Sagittarius which is squared by Moon in Virgo and sextiles Uranus. So, this person’s pretty open-minded, big picture oriented, futuristic-leaning, enjoys new experiences that broaden his mindset. His thoughts may not be in line with his feelings, how he responds and react to things in general – in a critical, analytical manner. Gravitates towards being functional and practical rather a dream, a concept or idea that is unproven. There’s the freedom to speak, think which can be independent. Interesting ideas and leans towards non-mainstream ideas and concepts.
So, when transiting Neptune hits natal Mercury with a square, what may happen? Without considering house rulerships, this could be a confusing time for the person. Mercury is in a mutable sign so there’s flexibility and adaptability in thinking. With Neptune, the thinking and communication could be all over the place. This can feel rather disconcerting to the person where he’s prone to specifics and rational discrimination of what’s happening. The question is: how do you analyze Neptune?? With Uranus, there could be flashes of insights or brilliance during the transit as well. With a floodgate of mental impressions being opened, on top of those intellectual brilliance which ignites the brain and mind, our man is going through quite a lot.
What if Mercury is located in the natal 12th house and rules MC, 6th, Moon in 9th and Pluto conjunct MC? He might feel that his intuition has opened up, feel alone and even potentially feel unable to act and control his thoughts which could affect his power and control over his future and career, his usual emotional comfort from his knowledge and daily work routine/health.
How can our protagonist bring out the positive side of this hard Neptune transit? Firstly, as Mercury is located in the 12th house, perhaps he could write down his mental impressions and evaluate them or get someone who he trusts to help to sieve through them. If communication/contracts/commercial deals are part of his career and work, it’s best to triple check them before documents are signed or finalized. Or have someone independent to do it. How about incorporating creative writing on overseas travelling or esoteric knowledge (Moon in 9th) as part of work/career (6th and 10th) alone (12th)?
As it’s a hard aspect, it’s best not to make major decisions during this time unless the decisions are based on reality and the plans can really be realized instead of being realized in imagination.