It's so simple and yet so effective; reducing the 'friction factor' of asking a question to almost nothing. The questions I get asked when I pass someone in a corridor, or when I'm straightening the newspapers or doing any one of a dozen small jobs I intentionally space throughout the day to give me reasons to be walking about the library being visible and ready to be asked, often start 'I wasn't going to bother you but as you're here' or 'I don't suppose you know, but' or 'this is probably a really dumb question, but' - which is to say, exactly the kind of things that people might *not* ask if they had to come up to a desk, or come across to my office, however open-door and accessable I try to make myself. They're very often questions I can answer, so that people get whatever it is they need, whether that's directions to Abbey Road or advice about how to untangle an unclear citation, or where such-and-such a book is shelved. It also gives me some clues about what people are not asking, which is useful when I'm thinking about signing, or 'how do I...' tips, or reminder emails. And the newspaper rack gets straightened. In short - everybody wins.
* Photo by Ben Werdmuller, used under Creative Commons, with thanks.