WHEN freed American hostage Terry Anderson first spoke with the press in Damascus, Syria, last week, his thanks included "that wonderful man a tall, dark figure standing behind him.Italian-born Giandomenico Picco, special envoy to United Nations Secretary-General Javier Perez de Cuellar, had been quietly conducting talks with captors and governments for many months. It has been a risky and largely thankless job. Mr. Perez de Cuellar, who also played a central role, vows the work will continue until all hostages are free. "The hostage game has been over for a long time," observes Augustus Norton, a political scientist at the United States Military Academy, "but I think a lot of this has to do with the simple doggedness and sincerity of Perez de Cuellar and his staff ... and the energy they invested in this." Energy is the right word for almost every diplomatic effort under way at this once-staid institution. No longer does superpower rivalry block every UN effort to act. Even the 166 colorful flags (including those of seven new members) flapping vigorously in the breeze on First Avenue seem to be standing a bit taller. "A growing sense of urgency seems to be building about the need to work together," Perez de Cuellar notes. This week the El Salvador peace talks are expected to move from Mexico to New York where it is hoped that the UN's ample resources may give a "final push" to negotiations, says UN mediator Alvaro de Soto. Meanwhile another special envoy, Cyrus Vance, just ended his latest round of discussions in Yugoslavia in preparation for the possible dispatch of a UN peacekeeping force there. The Security Council stands ready to send troops if a lasting cease-fire is reached and the secretary-general gives the word. The General Assembly is at that point of the year - about one week from its finale - when resolutions are passed with near-laser speed. Later this week the Assembly is expected to recommend a partial easing of sanctions against South Africa in limited people-to-people contacts. African National Congress President Nelson Mandela and the General Assembly's Special Committee Against Apartheid have both urged the change. At the UN these days much of what was once considered each nation's own business is now everybody's business. Human rights and the environment are prime examples. On Nov. 29 the Assembly unanimously came down hard on Myanmar (Burma), expressing dismay at its continued military rule and refusal to yield power to an elected parliament. At a ceremony last week unveiling a new painting that will serve as the official poster for the UN Earth Summit in Rio next June, Perez de Cuellar said he sensed that most nations now realize that sovereignty must be reconciled with the "need for global stewardship." Along with the new energy permeating most UN problem-solving efforts is a growing awareness among many UN officials of the powerful role the public can play in building pressure for change. Last week, for instance, Maurice Strong, secretary-general of the upcoming UN Conference on Environment and Development in Rio, launched an effort to get every world citizen to sign an "earth pledge" to help make the planet "a secure and hospitable home." An ebullient man of infectious enthusiasm, Mr. Strong insisted accountability is key to Rio's success: "Every single person has a stake in what their leaders do or fail to do.... Summits can't sit by themselves in the clouds. They have to have a base."