Yes and yes. I did both. It would depend on the bunny and what I had ongoing at that time.
If the bunny was directly episode related or, as happened a few times in NCIS, came from a preview of the next episode, then I would make sure it was written and posted before the episode actually aired - and I always made a point of saying that in the story header.
If it was a story I was writing for someone, then I would tend to write it straight away. Ditto, if I was writing a long story and I had a bunny I knew would be 'short and sweet' I would write the short story and go back to the longer one.
Otherwise I would jot down the idea, sometimes it could be as simple as just writing the title, sometimes I'd write a few words or a couple of lines, occasionally it would run to a couple of paragraphs. However, for the most part I tended to rely on my memory.
19 – When you have bunnies, do you sit down and start writing right away, or do you write down the idea for further use?
20 – Do you ever get bunnied from other people's stories or art in the same fandom?
21 – Sequels – Have you ever written a sequel to a fic you wrote, and if so, why, and if not, how do you feel about sequels?
22 – Have you ever participated in a fest or a Big Bang? If so, write about your favourite experience in relation to one. If not, are there any you've thought about doing? And if not, why not?
23 – When you post, where do you post to? Just your journal? Just an archive? Your own personal site?
24 – Betaing – How many betas do you like to use to make sure there aren't any major flaws in your fic? Do you have a beta horror story or dream story?
25 – Music – Do you listen to music while you write? Do you make playlists to get into a certain 'mood' to write your fic? Do you need noise in general? Or do you need it completely quiet?
26 – What is the oddest (or funniest) thing you've had to research for a fic?
27 – Where is your favourite place to write, and do you write by hand or on the computer?
28 – Have you ever collaborated with anyone else, whether writing together, or having an artist work on a piece about your fic?
29 – What is your current project or projects?
30 – Do you have a favourite fic you've written? What makes it your favourite? And don't forget to give us a link!