I get that all the time. Of course those teeny ideas don't usually translate into anything. I'm not the type who can write five books at once. And it's not like I see/hear these things and have an automatic plot. Mostly it's just the thought of that one line would make a really great scene or that person would make an awesome character. Sometimes, they already are characters but I want to remake them for my own.
Like yesterday, I watched a Brit teen movie called Angus, Thongs, and Perfect Snogging (what a title, right?). The love interest in this movie, one Aaron Johnson, was just oozing teen dreaminess. I wanted to immediately sit down and write his story- but my way. Not the movie way, and not his real life way, my way. Dream up my own teen love story full of angst and mishaps and craziness but where the girl gets the boy in the end.
The point here is to always keep your eyes open for ideas. Watch the world around you, read like crazy, and even movies can give you inspiration for your own stories. Even though a lot of these ideas haven't translated into stories for me- yet- I know they will one day. Or at least bits of stories, ideas for stories, and characters for stories. So the next time you walk by someone and think, wow that person has some very interesting features... take notes. Because you might want to use them one day.