THE TALL, mustachioed figure was often just a few paces behind Warren Christopher during the tense negotiations for the release of the hostages. And in the emotional moment when millions of Americans caught first sight of the former captives as they came of the airplane in Algiers, his was one of the faces on which the 52 Americans planted their grateful double-kisses.

While the hostage drama has rightly focused on Deputy Secretary of State Warren Christopher as the giant in the negotiations, one unsung hero was the mustachioed man accustomed to dwelling diplomatically in the background: U.S. Ambassador to Algeria Ulric S. Haynes Jr.

Not only was he a key part of the negotiating team -- an important facilitator -- but as U.S. envoy to Algeria for the last three years, he was vital in smoothing out Algerian anti-Americanism that ultimately would make it feasible for Algeria to play the difficult and delicate role of middleman.

"Rick gave us incredible support," said Arnold L. Raphel, a member of Christopher's negotiating team. "He made the decision not to be involved in direct negotiations. But he spoke fluent French. No one on the neogtiating team spoke French. In terms how you cooperate with Algerians. Rick was very useful and got us great access. In a crucial area, he was our entree to the Algerians. His was an invaluable role as a go-between."

But Haynes' greatest work may have been accomplished long before the Americans were seized by Iran, and long before last November, when it was announced the Algeria would serve as Iran's intermediary with Washington. Haynes had played a quiet diplomatic role in a number of unusual developments between the United States and the socialist government in Algiers.

Part of it is a question of timing. For years, Algeria was a thorn in the United States' side, the landing pad for airplane hijackerrs and terrorists, and a haven for such 1960s American militants as Eldridge Cleaver.

When Rick Haynes went to Algiers in 1977, Algeria was under the leadership of a hardliner, President Houari Boumediene, and although the two countries maintained economic ties, political relations were at a low point -- strained and tense. Haynes, called "incredibly personable" by negotiating team member Raphel, is credited with markedly improving the environment for U.S. interests.

When Boumediene died in 1979, President Chadhi Bendjedid took over and relations continue to improve, partly because the non-aligned Algerians want to court the United States, without alienating Moscow, in an effort to resolve the long dispute over American support of Morocco, which has been engaged in a lingering war in the western Sahara desert against Polisario guerrillas headquartered in Algeria. Any progress in settlement of the Saharan dispute must lie partly with the quiet diplomacy of Haynes, observers say.

"He is seen as a very strong ambassador," said George Dowly, formerly with the State Department and now with the Civil Aeronautics Board. "When relations on the scene improve, the ambassador on the scene usually gets the credit."

In one sense, as a black ambassador, Haynes probably played the same kind of role that Andy Young once played as the United States ambassador to the United Nations -- he brought a certain credibility to United States' dealings with the Third World.

While there is no magic in being black, many blacks tend to go into the diplomatic service with a certain degree of sympathy and are prepared to take a country like Algeria seriously, so it probably was easier for Haynes to have greater sensitivity toward Third World concerns.

The more typical white American view was most lilkely the French one -- it was a socialistic Third World government that is suspect because it is non-aligned and not clearly pro-West.

Haynes, 48, of Columbus, Ind., a former specialist on Africa on the National Security Council staff during the Johnson administration, was graduated from Amherst College, holds a degree from Yale Law School, and in the most recent years, before taking the ambassadorial posts, was with Cummins Engine Co. as vice president for the firm's Middle East and African concerns. He lived in Iran for several of his years with Cummins. After serving with the United Nations in Europe and Africa, he worked for the Ford Foundation. From 1975 to 1976 he served on the National Security Council staff.

Thus, when the Iranians chose the Algerian government to play the difficult role of adjudicating the monetary guarantees required before hostages were released, and the Algerians agreed to accept being placed in that tough position, they set into motion the resolution of a problem that had confounded and humbled the United States, tearing its emotions apart as had no event in recent memory. The heroes and villains of this long drama are fast becoming legendary. It's nice to know one of the good guys was a diplomat who wore a black hat.