At work I hardly ever start a conversation, it just doesn't happen. If they start one, I can hold conversations with them and I think I manage to bluff my way through fairly well. After all these years of practice, people think I'm just shy and very reserved. It usually feels awkward and fake and halting in my head and I am kind of fascinated how organic and natural other peoples' chats and conversations seem. I do hesitate to join in, I don't know how or when it's appropriate to do that. It is only on very rare occasions that I feel that a real, genuine interaction has happened and flowed naturally with someone who I don't know really, really well. So I do tend to keep quiet, mainly to save myself the stress and irritation.
I talk very easily with my family and with my friend, but even with them, if I'm tired I can barely string a sentence together. I have been known to grunt or mutter monosyllabically when my mum's telling me about people and things on our weekly phone call. If it's been a busy, noisy, annoying day at work, sometimes just don't have the energy left to put into it. Sorry mum.
I talk to myself a fair bit, though.