Perhaps the most obvious interpersonal skill would be teamwork. Being able to work in a group with synergy, great communication, and an equitable sharing of ideas not only makes for a more comfortable environment; but also speeds up productivity. When things really start to flow, ideas are abound and tasks are being delegated through polite and energetic communication, then everyone benefits.
Politeness, listening skills, and voicing your own ideas clearly and openly are all signs of effective teamwork. You scratch my back, and I’ll scratch yours!