"is America going to be a player or a spectator?" acting Egyptian Foreign Minister Boutros Ghali asked a luncheon guest before the Cairo peace conference got under way last week. By the end of the week, Prime Minister Menahem Begin of Israel and President Anwar Sadat of Egypt had left little doubt they expected the Americans to come with their helmets.

Though perhaps less visible than before, the U.S. role will be played at the very top - among Sadat, Begin, President Carter and other heads of government. Events suggest that in large measure it will come down to defining with high specificity the appropriate international setting for a Palestine homeland.

The big move focusing new attention on the American role was Begin's sudden trip to meet with president Carter in Washington Friday. Begin took with him the outlines of proposals the Israelis expect to aly on the table in the formal negotiations now under way here. The Israeli prime minister clearly wants the President's support of his position.

Sadat intensified the focus on Washington of what has become a major part of his diplomatic effort - press and television interviews. In an interview with this columnist just before the Cairo talks began, Sadat stressed the American role and the level at which it should be played.

He recalled previous statements that "the U.S. has 99 cards of this game in its hand." He added that after his trip to Jerusalem "this role has been confirmed. It has not at all been minimized."

He left no doubt that he did not regard the Cairo talks as much more than a facade for negotiations among the heads of government. He had had advance word of BEgin's visit to Washington and held out big hopes for the results. He said in general that big decisions had to be made by political leaders, not technocrats.

As to the substance of what the United States can do, there are many possibilities. Both the Egyptians and the Israelis are counting on the United States to prevent the russians from being ttotal spoilers. Indeed, Sadat has said of his visit to Jerusalem: "I gave President Carter a white horse. I gave the Russians a cold knife."

Additionally the United States can encourage some of the more moderate Arab states to back or join at some later stage the peace talks now under way. Secretary of State Cyrus Vance initiated moves in that direction during his swing through the Mideast last week.

By far the biggest opportunity, however, lies with the issue of a homeland for the Palestinians. The Cairo negotiations will essentially be a trade whereby the Egyptians concede peace to Israel in return for territorial concessions in the Sinar desert, and Israeli acceptance of some kind of Palestinian presence on the territories west of the Jordan River, which they seized in the Six-Day War of 1967.

So far, Egypt and Israel seem to be far apart. Sadat has indicated that the Palestinians on the West Bank have to have self-determination, including the right to set up their own state. But he has been systematically undermining the claims of the Palestine Liberation Organization to represent the West Bankers.

He told others that the leader of the PLO, Yasser Arafat, was "weak" and "helpless," and told me that Arafat was under "pressure and under strangulation from Syria." He called into question the decision of the 1974 summit meeting of Arab leaders, which established the PLO, rather than King Hussein, as the sole representative of the Palestinian people.

On the Israeli side, Begin has repeatedly said his government would not accept any foreign sovereignty over the West Bank - especially PLO rule. But Foreign Minister Moshe Dayan has developed a plan for "functional" concessions - turning over, say, police, sanitation, education and other governmental responsibilities to local authorities. Obviously functional concessions become the first step in a process that would eventually lead to territorial turnover.

Between these two positions there is an obvious mediating role for the United States. President Carter has talked extensively about a Palestinian homeland, but never defined exactly what he had in mind. My impression is that both the Israelis and the Egyptians would like him to come down very hard on something he has so far only intimated. That is the view that the Palestinian homeland, far from being a loose cannon careening back and forth, should be closely affiliated with the Jordan of King Hussein.