People,
PLEASE, if you are really interested in learning quality Astrology, do NOT give into ignorant cook-bookish stuff like: "If you are a Sun in Scorpio, of course you are going to be deceitful", or, if you are a Gemini, of course you are going to lie. Such sentences in themselves are the biggest lies, and they just show how 'green' one is at the subject, and can be very misleading for other greenhorns.
Each and every sign is capable of deceit and so is every planet. Yes, even the reality-loving Saturn. When? When Saturn is fallen in a chart, it cannot live up to its ability to stay close to reality and remain straight.
Deceit itself is a very broad term and so prevalent today due to many reasons that it is the truth that is actually more surprising and needs to be researched more today.
By itself, Mercury, simply put, is communication and the intellect/ logic. However, when Mercury is debilitated (poorly placed), its logic becomes illogical, its thinking erratic and digressive and its communication (which is directly connected to one's thinking) accordingly is also affected.
So, don't just go ahead and say that the Sun or Mercury in a certain sign automatically means that the native is deceitful because by saying that you are only deceiving your own self and other novices.
If you know yourself to be deceitful or have been so, look at your #COMPLETE chart with all its connections, placements, aspects, strengths and weaknesses.. Also, never listen to people that say that signs are less important or houses are even less important because you are clearly dealing with a complete greenhorn, who is clearly given to either ignorance or prattling.
Often, you will read that Neptune's aspects are most common when it comes to deceit. Actually, that too is a little harshly put. What happens with Neptune (the Lord of the oceans - Poseidon) is that it keeps flowing (typical of water) and is therefore treated as sort of the opposite of Saturn, which stands firm or stays put firm as a rock. Water is also often connected with spirituality and imagination (due its nature of natural flow and being boundary-less), and therefore the associations with idealism and escapism. Also, with compassion.