I'm a sleepwalker (and a sleeptalker). I've had episodes my entire life, ever since I was a small child, and there doesn't seem to be any particular pattern to them: episodes are quite sporadic (sometimes I'll have several in one week, sometimes there won't be one for months) and don't seem to be set off by any health issues, change in sleeping pattern or lifestyle, or anything out of the ordinary. They just happen.
When I was quite young I sleep-walked around my entire neighbourhood, in my pyjamas, and ended up in my parents' garden, with muddy feet but otherwise none the worse for wear. Fortunately it was summertime, although I suspect I would have woken up if it had been cold outside. This was over twenty years ago and despite living in a small town and in a quiet neighbourhood, my parents locked the doors even then and I still managed to get out. (I left the door open, though -- thank goodness we didn't have any pets back then!) Another time I came downstairs from my bedroom, sat down beside my father on the couch, and carried on a conversation with him (that I had no recollection of come morning). Once, when my younger sister and I shared a bedroom, our mother came in and heard us talking in gibberish to each other (we both talk in our sleep); we weren't speaking English, but whatever we were saying to each other apparently made sense in our minds!
Now that I'm older, incidents are less common and somehow more embarassing. During my first year of university, when I lived in residence, I dragged my roommate's chair across the floor of our room and put it beside my bed so that I could climb up on it to retrieve (non-existent) stuffed animals from the (purely imaginary) shelves over my bed. I was very startled to wake up in the morning and see the chair sitting beside my bed -- I remembered dreaming about climbing to reach my stuffed animals, but didn't think I'd actually tried to do it!
Living with my boyfriend just makes things worse, because I always wake him up and he gets a big kick out of my sleepwalking/sleeptalking. I once went into the bathroom to take my shower, started the water, started stripping, only to wake up and realize it was only 2am! Another time I poked my boyfriend in the nose (waking him up), giggled and turned away. I also frequently leap out of bed because I "see" spiders (or other insects) either in my bed or dangling from spiderwebs over my head.
Now that I'm older, I have a tendency to wake up in mid-action (except for the chair incident), so I typically stop myself from doing incredibly stupid things, like turning the stove on or tumbling down the stairs. My boyfriend also usually wakes up with me, at least partially, so if I start doing something out of character or odd for that time of night, he'll stop me. I'm lucky in that I haven't had any bad accidents or incidents (which is amazing, because I'm remarkably clumsy even when I'm fully conscious!), but I'm uncomfortable sleeping in new places because I'm afraid of what I'll do. And I hate, hate, HATE being told of the stupid things I did while I was asleep -- it's almost always intensely embarassing.