Independent counsel Lawrence E. Walsh is expected to seek to interview President Reagan about the Iran-contra affair, probably this fall, before pressing ahead with criminal charges against some of his former aides, according to informed sources.

Walsh will not talk about the progress of his inquiry, but sources familiar with the investigation say he still must collect a considerable amount of evidence before obtaining a broad conspiracy indictment against Lt. Col. Oliver L. North, the fired National Security Council aide; Rear Adm. John M. Poindexter, former national security adviser, and others.

Asked when he was appointed last December whether he would seek to question the president, Walsh said he would "talk to anyone necessary" in the executive branch to secure the information he needed.

Indictments are now expected in late October or November at the earliest. In addition to the work still to be done by his office, the sources said Walsh and his prosecutors have decided to wait until after the Senate-House Iran-contra committees make their final report. Committee officials say they hope to do this by early October, but they say it is "a massive job" and could take a bit longer.

Before Walsh asks the grand jury to return any conspiracy indictment, however, sources say it is virtually obligatory for him to try to question the president about what he knew of the alleged misdeeds. Otherwise, the prosecutors risk being blind-sided later on by defense claims of presidential authorization.

In his nationally televised speech Aug. 12, Reagan accepted full responsibility for "the Iran-contra mess," saying he had been "stubborn in pursuit of a policy that went astray." But he did not address the questions still surrounding his actions and those of his aides in selling arms to Iran and in the creation of a covert military support program for the contra rebels in Nicaragua.

For the past eight months, Walsh and his prosecutors have been pursuing the premise that the secret network North set up to carry out the arms sales and funnel supplies to the contras amounted to a criminal conspiracy to defraud the government. The questions likely to be put to Reagan include: What did he say or do that led his aides to believe they were complying with his wishes? Would he have approved of the diversion of arms sales money for the rebels? Did anyone ever ask him about it? What does he think of the lies that were told to Congress during the course of the operations -- and the hurried destruction of records when their disclosure was imminent?

Reagan could decline to answer the questions, but his three predecessors, Gerald R. Ford, Jimmy Carter and Richard M. Nixon, responded formally to questions about controversies during their administrations. Ford testified before the House Judiciary Committee in October 1974, defending his pardon of Nixon in the Watergate scandal. In 1976, after he left office, Nixon submitted sworn answers to questions posed by the Senate Select Committee on Intelligence in its probe of CIA and FBI operations. In December 1980, Carter was interviewed under oath at the White House by Justice Department investigators looking into the relationship between his brother Billy and the government of Libya.

Other unfinished business facing Walsh and his staff, sources said, include reexamining CIA witnesses, filling the gaps created by the destruction of White House records by North and others, questioning hard-to-reach Israeli witnesses and examining Swiss bank records that are still being processed for release to U.S. authorities.

Investigators for Walsh's office were reportedly surprised when Alan D. Fiers, chief of the CIA's Central American task force, told the congressional committees Aug. 5 about a "policy" plan for Central America that the late CIA Director William J. Casey told Fiers to draft in November 1984, shortly after enactment of the Boland amendment prohibiting military aid to the contras.

Fiers said Casey told him to "put down on paper for me a policy where you think we ought to go." Casey told him he would get the policy approved and shortly thereafter, instructed Fiers to furnish a copy to Col. North.

Fiers had not told Walsh's investigators about the document.

The congressional immunity granted to North, Poindexter and others for their testimony on Capitol Hill has created other potential problems, stemming from the determination of Walsh and his staff to ignore those portions of the Iran-contra hearings and thus avoid being "tainted" by such knowledge. Nothing that North, Poindexter and other witnesses granted immunity said can be used against them, unless they perjured themselves.

It is possible, therefore, that the special prosecutors -- except for a "tainted" monitoring team that cannot discuss the hearings with the others -- do not know that Poindexter destroyed a secret "finding" that he said President Reagan signed in December 1985, approving an arms-for-hostage operation.

It is also possible that they do not know that North, as he has testified, prepared five "diversion" memos, concerning Iranian arms sale proceeds, for Poindexter to submit to the president with boxes for Reagan to "approve" or "disapprove."

The question of perjury by witnesses granted immunity poses another problem. Under relevant court rulings, Walsh and his prosecutors have taken the position that they cannot look at such congressional testimony, even at the post-indictment stage, because it could impermissibly influence their trial strategy. As a result, sources say, any perjuries that may have been committed under the limited grants of immunity will have to be put aside by Walsh's investigators until after the conspiracy case is thrashed out in the courts -- even though the perjuries might constitute overt acts in furtherance of the conspiracy.

"There's no sense jeopardizing a substantive case in order to pick up a perjury charge," said one source. "The statute of limitations on perjury is five years. They {prosecutors} can come back to that if they ever have to."

Similarly, Walsh and his staff are said to be apparently unconcerned about any gaps caused by deliberate ignorance of key portions of the congressional hearings. After a long investigation such as this, a law enforcement source said, there is less room for big surprises.

From the outset of the investigation, Walsh has been pursuing a broad case alleging conspiracy to defraud the government in the course of carrying out the secret arms sales to Iran and diverting profits for the Nicaragua contras and other purposes. Supporting charges, which would also constitute overt acts in carrying out the alleged conspiracy, could include obstruction of justice, falsifying government records, and destroying government property.

The federal grand jury working under Walsh's direction will resume work this week after a two-week vacation. Sources say it is not likely to return any indictment until late fall unless it is deemed necessary to move against a recalcitrant witness in the weeks ahead. The sources also say it is possible that the investigation could grind on into 1988.

"This is a case where you're working, essentially, with hostile witnesses," one source said. "And anytime you're dealing with a legal problem, it always takes longer than you think."Staff researcher Michelle Hall contributed to this report.