Little things often affect what we do in large ways. It's not because they have any innate power of their own, or even that they (often) combine to shake the earth that you walk on. It’s because small things are always indicative of larger works in progress, and we are actively looking for those indications. They resonate with us.
Take, for example, a previously happy married couple who are now bitterly angry over backward toilet paper rolls and improperly squeezed toothpaste. Are these things truly the cause of their unhappiness with each other, or are they simply indicators that show a larger lack of respect for each other's feelings, and an inability to effectively communicate?
What about that small twitch of the mouth or subtle way of moving that seems to signify a character? Obviously, the affectation doesn't really define that character -- there's no detail there, no meat -- but it is a strong indication of who he is and what he's about. There’s a story there, and it gives us gut feeling about what to expect from that person; it makes us want to know more. The simple fact that the quirk stands out hits our body language translators like a fist, turning that small, potentially insignificant thing into a beacon that can make for a truly electrifying performance.