As a young movie actor, Ronald Reagan was frequently cast as a man of action or a homespun hero on whom his friends could depend. Last week, for those still interested in the drama of the Iran-contra affair, Reagan starred in a new role as a contrite and aging president who wants to go out a winner.

For a politician whose career seemed on the skids, Reagan gave an adequate performance. He did not say whether, given the chance, he would have diverted arms profits to the contras. But he did manage to walk what he apparently believes is a fine line between groveling and apologizing to the American people for betraying their trust. He also acknowledged "that there's nothing I can say that will make the situation right."

Unfortunately for those who share Reagan's arms-control goals or his high opinion of Supreme Court nominee Robert H. Bork, the performance was a carefully crafted one-night stand in the Oval Office that cannot safely be repeated outside the protective shield of a television studio.

As Reagan begins a long California vacation that few would begrudge him, he leaves behind a sheltered presidency in which isolation approaches an art form. Reagan has given so many contradictory explanations of unresolved issues in the Iran-contra affair that he dares not comment further without a script. When the president is dispatched to the White House briefing room to announce a new appointment or restate an old policy, his handlers warn him against answering even friendly questions.

This is sad, for Reagan and for the country. On his best of days at the height of his popularity, he was a high-risk performer in the briefing room who rarely allowed a fact to get in the way of a stray thought. But part of his considerable charm was that he usually stood his ground with the media and said what was on his mind.

Those days are gone, probably forever. Modern presidents are often hidden for a few months when policies go sour, but Reagan is breaking even his records for prolonged inaccessibility.

Ironically, the isolation has become endemic under the reign of a new White House team supposedly dedicated to openness instead of the secret and devious behavior characteristic of aides during the Iran-contra affair.

White House chief of staff Howard H. Baker Jr. and national security adviser Frank C. Carlucci are respected public servants, wise in the ways of Washington. When they took over from Donald T. Regan and Rear Adm. John M. Poindexter, it was widely expected that Reagan would be released to resume normal communications.

This has not happened. Instead, the president is more isolated and inaccessible than at any time in his political career. Under Regan, he averaged a news conference once every two months; under Baker, the average is one in three months and likely to worsen. White House spokesman Marlin Fitzwater treats even the mention of a news conference as a joke.

News conferences are not, of course, the only measurement of accessibility. By other yardsticks, Reagan is even more isolated in the Howard Baker era. For instance, Reagan gave repeated interviews when James A. Baker III and Regan were his chiefs of staff. Now he gives none, except for occasional chats with journalistic boosters.

Other presidents unwilling to face the media have sometimes substituted panels of citizens to ask them questions. Even this dubious device cannot be safely utilized by Reagan in his present state of mind.

On July 27, campaigning in Wisconsin for his "economic bill of rights," Reagan wandered away from his script in a speech at Port Washington. This is what he said: "You know, a president some years ago was talking about government and its power, and he said, 'If the people don't know enough to run their own lives, where do we find a little select elite that cannot only run their lives themselves but the people's lives for them. And that is government?' Well, you can't find them."

Okay, so Reagan gave a swell performance last Wednesday. But the Great Communicator functions only on national television when he can read what has been written for him on a TelePrompTer. Left to his own devices, the Great Communicator no longer exists.

Reaganism of the Week: Asked last Monday in the White House briefing room why he was afraid to answer reporters' questions, the president said, "Who could be afraid of a sweet bunch like you?"