'L'esprit d'escalier': literally, the spirit of the staircase. Figuratively, all the imaginative comebacks and replies that you come with too late.
I always come up with a hell of a reply right after I've quit the conversation, or left the place, and it's always a stunner, and I always wish I had said it instead of tripping over my tongue and rolling words helter-skelter through my mind and out my mouth.
If I'm too nervous I tend to come off as a stutterer, or worse, someone with not a word in their vocabulary.
I'm so articulate in print that I sometimes hate myself for overpanicking.
The overpanicking tends to mellow down to nearly panicking to zero panicking after a while. I suppose that when a conversation with a stranger, when you have no idea what is going to be asked or will be discussed, will in the beginning be difficult. As you get through the conversation, you begin to get an idea of what they are talking about, or want to know, and so you relax.
The level of panic I may feel also relates to the relationship between myself and the other person. A person I know very well I wouldn't panic about at all, but a total stranger... mm. If I'm the one asking the questions, I feel better since I have an idea of how the conversation will wind its way. If I'm being asked, I tend to clam up.
When I write, I write for a faceless reader. The faceless reader has no opinions on my writing, will not express any opinion (if at all) when I am done with my writing. The faceless reader is, I think, the closest to me- they will never judge me or disagree with me, will think the world of me... right up until they take on a face.
One person put it best, on the Eloquent in My Native Language section of Troper Tales:
I put a random twist on it; I don't speak anything other than English, but while I have a great vocabulary and get my point across very easily on paper, I stumble over my words and sound like I'm mentally deficient when speaking. Apparently my native tongue is writing.
Incidentally, my response to this was: I feel so happy that I'm not the only terrible speaker/good writer around. The difference is so bad that when my English teacher said that for a class debate he'd pick people who don't argue well, he immediately added that I didn't count since I argued well in writing.