As much as it pains me to admit, I am guilty of this. What is stonewalling you ask? It can show up in a couple different ways like when someone refuses to participate in the conversation or mid-sentence they just change the subject. Other ways of stonewalling include the silent treatment or my personal favorite, when asked if something is wrong (because clearly there is something wrong) I simply respond "nothing" (see my post about Miscommunication).
For me, stonewalling started out innocently, usually by trying to avoid conflict or calm myself down and keep from stressing out, or sometimes I honestly didn't know how I felt at the moment or what to say so it was a defense mechanism of sorts to just stay silent.
The impact that stonewalling has though is not so innocent, quite frankly, its passive aggressive in that we think the other person should already know what's wrong or what they did wrong so we shouldn't have to tell them. That leaves the other person feeling ignored, misunderstood, invalidated or just plain hurt. Left unattended for too long, it will certainly lead to resentment.
Many times I would have already played out how I thought the conversation would go before even having it and then I would use that as justification for not having the conversation at all (see my post on Assumptions). It has taken me a long time to realize I was going about it all wrong.
If we are going to have successful relationships where communication flows freely then we have to start assuming the best of our partner, every time. Stop thinking we know what they are going to say or do because they might surprise you. I know I have been surprised time and time again with the patience and willingness to learn from each other and grow together, the thoughtfulness that my significant other has shown me has changed my whole way of thinking and interacting.
I haven't yet mastered this completely but I am definitely well on my way. Hope this helps someone else as much as learning it for myself has helped me!