In many modern astrology books you might have read, you will have noticed that very often, they offer the same interpretations for, say, Mercury in Scorpio, Mercury in the 8th house and Mercury in aspect to Pluto. A natal chart is confusing enough as it is and there's no need for us to add even more confusion to it.
As I see it, the planet's way to behave is determined by the sign it is in. The planet's sign placement will determine the environment it is born into and there's no escape from that. The planet, naturally, has some innate qualities and based on the sign it is in, we can judge whether the qualities of this sign will allow the planet to function according to its nature. Mercury, among other things, stands for objective thinking, the need for proof and logic. So, does Mercury like water? No. Logic and sentiment are incompatible. Mercury in Cancer, therefore, will find it difficult to be objective about things, because the emotional nature of its sign will impose itself on its views and beliefs, leading to an increased subjectivity.
If this Mercury in Cancer is placed in the 6th house, no, it doesn't automatically acquire any Virgo qualities. First of all, this planet/sign association is open to debate, to say the least. Furthermore, houses do not give qualities to planets. The planet in question affects the affairs of that house for better or worse. Finally, the house placement will allow us to determine whether that particular planetary energy will be able to manifest strongly in the native's life or not. If the house is angular, for example, the planet is much more likely to be immediately evident.
As far as aspects go, Mercury in Aries is not the same thing as Mercury in aspect to Mars. Mercury in Aries, although it is temperamentally unsuited to fire because of the passion involved which often leads to hasty decisions and impulsive thinking, will have no option, exactly because it is placed there, to behave in such a manner. A different Mercury in aspect to Mars, even if we disregard Mars' sign, will also have no option but to behave according to the nature of the sign it is in. An aspect from Mars will be an external influence that will interfere with the way the planet is predisposed to behave, but it will not magically transform it. If Mercury is in Virgo, naturally inclined to objective thinking, it may, at instances, succumb to impulsiveness, but will probably regret it soon afterwards because it's not the way it wants to behave. What Mercury in Aries may find natural, for Mercury in Virgo in aspect to Mars, this will probably be a problem.
