There are times when one sits back and analyzes a situation, so that correct thinking has occurred prior to talking about it, but if you wait too long to communicate it, your imagination may start playing tricks on you. When it comes to relationships, we tend to not want to bring up an uncomfortable topic to keep things calm/status-quo, not rock the boat and allow ourselves to over analyze and sometimes suffer needlessly.
One of the greatest aspects of my relationship with SirW is the open door communication policy. My not knowing about the world of D/s prior to my getting together with Him has made it necessary for Him to explain a lot of things to me that would otherwise (in a vanilla setting) be questionable. SirW greets every question or concern i have head on and calmly explains things to me without taking offense to the topic and i totally appreciate being able to approach Him with anything/everything.
This has also been applied sexually. It may seem unromantic to direct your partner in the middle of having sex/making love and perhaps the direction would be better taken prior to or after, but in any case, i feel it's important and very beneficial to communicate to your partner how you like to be touched...show your partner how to get you off, what you like touched, the exact way to rub whatever body part does it for you. If you are turned on by your elbow being pinched *giggles* tell them!
This is especially good in the beginning, when you and your partner are getting to know each other sexually. Everyone is different. What one person likes, another may not enjoy, so learn everything you can about your partner and don't be afraid to talk about experimenting. Nothing should be taboo when it comes to sexual intimacy, no judgement should be passed - if you do not like something, speak up...your partner is not a mind reader and with some loving direction, you can both be fully satisfied if you open up and show each other how your specific body works.
I do not know if there's such a thing as over-communicating, but there are so many moments when communication is lacking. Nurturing a great relationship takes time and energy, so it's worth all the effort it takes and allows healthy growth in return.
What ever the topic is, you will find that you will typically feel a sense of relief when you get it off your chest and find that you may have worried for nothing or that the other person was thinking the same exact thing.
No one knows how much time they have on Earth...if you do not communicate unsaid things, you could end up living with regret, so do not hesitate, do yourself a favor and get to it!
You may never know everything about your partner, nor should you want to...some mystery is always good, but the important things need to be communicated in a timely manner or your imagination will fuck you up! *smiles* We are our own worst enemy when it comes to our imagination and we do it to ourselves - we torture ourselves in essence - stop it! Communicate!