SAN SALVADOR, AUG. 22 -- "Everyone comes to me with the same question. What happens, they say, if Central America doesn't live up to the agreement we signed in Guatemala?" said Salvadoran President Jose Napoleon Duarte.

"But I'm asking, what happens if we do live up to it? We'll have peace!" Duarte said last night.

In his first lengthy interview since the five Central American presidents signed a preliminary peace plan in Guatemala Aug. 7, a newly invigorated Duarte pledged to carry out all terms of the accord and to press other nations to do the same.

The plan seemed to falter this week, however, as the five Central American foreign ministers failed to move beyond procedural matters in a two-day follow-up meeting. Honduras also expressed reservations, fearing that the pact could push thousands of Nicaraguan rebels, known as counterrevolutionaries or contras, to seek refuge in Honduras if the accord stopped U.S. aid to them.

Although he was crucial to the plan's approval, Duarte will not be central to its outcome. That hinges on the willingness of Nicaragua's government to move toward democratic freedoms and of the contras to wind down their war.

Duarte, however, said he is determined to dispel mounting pessimism in Washington and among diplomats in the region that the plan is too complex and ill-defined to succeed.

Only last month Duarte, in his fourth year as president, was avoiding reporters. His popularity was at a low ebb, polls showed. His U.S.-backed Christian Democratic government was under attack as corrupt and bureaucratic, and the leftist guerrillas were showing new muscle in the countryside and creeping back into the cities.

But the Guatemala pact strengthened Duarte's hand in El Salvador by allowing him to draw international support toward his government, isolating the leftist guerrillas and placing the peacemaker's mantle firmly back on his shoulders.

In recounting his version of the long night of Aug. 6, Duarte said he offered the proposal that broke the deadlock between the presidents of El Salvador, Guatemala, Costa Rica and Honduras on one side and President Daniel Ortega of Nicaragua on the other. Duarte said his idea was that all the actions mandated in the plan should go into effect at the same time, on Nov. 7, 90 days after its signing.

The two main arenas of fighting in Central America are El Salvador, where leftist guerrillas are trying to overthrow Duarte's U.S.-backed government, and Nicaragua, where the United States supports rebels trying to oust the leftist Sandinista government.

"When we got to the meeting the foreign ministers had a draft of the plan, and every point was 4 to 1, with Nicaragua objecting. So I outlined my concept. Ortega went back into the corner to consult with his advisers, and when he came back he was ready to accept," Duarte recalled.

Duarte said he spent several hours during the meeting enumerating to Ortega the ways El Salvador believes Nicaragua is aiding the leftist rebels of the Farabundo Marti National Liberation Front. At the end of the session, Duarte said, he sought out Ortega to ask him if he intended to comply with the plan, which requires Nicaragua to end all support for guerrilla armies and enact broad measures to open its society to democracy.

"I asked him once and he said, 'Yes,' " Duarte said. "I asked him again. Then I asked him to shake my hand, and as we stood there with our hands locked I asked him a third time: 'Do you commit your personal honor to comply with this agreement?'

"He said, 'Yes.' I couldn't really do more than that!" Duarte said, laughing.

Before the meeting, the leftist Sandinista government of Nicaragua had steadfastly refused to endorse any agreement that obliged it to make domestic changes.

Duarte's contribution to the clause, specifying that all actions will take effect at once, apparently assured Ortega that he would not dismantle part of the Sandinistas' tight system of political control only to face continuing attacks by the contras.

The 11-point Guatemala plan calls for political dialogue between governments and their unarmed opponents; a return to full electoral democracy in each country; an amnesty for guerrillas; an end to foreign aid to rebel armies; a ban on the use of any nation's territory by guerrillas of another nation; and a region-wide cease-fire.

Duarte admitted that he had a change of heart in the weeks before the meeting. In June, Duarte cooperated with the Reagan administration by demanding to postpone the regional summit when it seemed that events were favoring Nicaragua.

But recently, he said, "I decided if we were going to have peace we had to have the political will to do it. We Central Americans had to start speaking the same language and we needed a coherent plan for all of us."

Yesterday, Duarte met with leaders of the Nicaraguan Resistance, the contra coalition, and persuaded them to accept the accord.

The meeting was part of an initiative by Duarte calling for simultaneous talks Sept. 15 between his government and the Salvadoran guerrillas, and the Sandinistas and the contras.

{In Caracas, Venezuela, the foreign ministers of eight Latin American countries met with the five Central American foreign ministers and formed a verification commission to oversee implementation of the Guatemala pact, Reuter reported.}