It was, appropriately enough, like opening night at the theater, an opening night when a new show is in town featuring an experienced actor in an unfamiliar role.

The show was "President Reagan." It opened here under bright lights in the ballroom of the Century Plaza Hotel to good notices and the whispered undercurrent of excitement that attends first nights.

The extras in this drama -- a press corps increasingly dominated by cameras and microphones -- played their parts, too. They asked questions and dressed for their roles. In a town where blue jeans are common at news conferences, suits and dresses were the order of the day.

Reagan is, above all, a performer, and he was playing a role today he had auditioned for for 12 years. He knew what to do.

When a Turkish reporter asked him about his commitment to Turkey, the president-elect diplomatically replied that Turkey and Greece are "the southern flank of our NATO line" and therefore vital to U.S. security.

When another reporter asked about a possible conservative revolt against Sen. Howard H. Baker Jr. (R-Tenn.), in line to become majority leader, Reagan turned politely to his closest political Friend, Sen. Paul D. Laxalt (R-Nev.), and observed that Baker's position is solid and that there is no move to replace him. Laxalt nodded, a bit grimly.

And when a questioner tried to pin Reagan down on what Vice President-elect George Bush's role would be, Reagan turned the question aside with the deftness he had displayed in the campaign debates.

"Governor, during the course of the campaign, Ambassador Bush said if he gained your confidence during the campaign he would have tons do do and, if not, he would be going to lots of funerals," the reporter said. ". . . Are you expecting him to have a major role?"

"No, he's not going to a lot of funerals," Reagan said with a smile and no details. "Maybe we will take turns."

As the extras laughed with him, Reagan added, "What are we laughing about?"

In Hollywood, even the most serious of undertakings has its comic moments, and "President Reagan" was no exception.

The low point came when press secretary Lyn Nofziger's prearranged ending to the first news conference went awry because of technical difficulties.

In keeping with White House practice, Nofziger had asked a wire service reporter, in this case Terry Hunt of the Associated Press, to end the news conference. Hunt tried, but Reagan, whose hearing has been diminished by an early movie accident, couldn't hear him.

At Nofziger's gesture, Hunt tried again to say "thank you, governor," this time into a microphone. But the mike wasn't turned on, and the show continued.

Finally, Nofziger flung himself in front of the podium and told the new star and his supporting actor, Bush, that their time was up.

But "President Reagan" is not fundamentally a comedy, and the show this morning had a serious purpose which it largely accomplished.

What Reagan wanted to do, he and his advisers agreed, was to play the part of Reagan the Reassurer to foreign allies and foreign foes, demonstrating that he was not the warmonger his opponent had said he was. He wanted, also, to reassure those Americans who didn't vote for him that he would be a reasonable president.

To reassure foreigners, Reagan relied on the foreign press, carefully answering their questions with generalities and largely avoiding the questions of the campaign press corps which had covered him and might be inclined to remember some of his earlier, more warlike, statements.

Aware, also, that television reporters live solely by on-camera performance, Reagan methodically worked his way through the network questioners who were covering this performance live.

In Hollywood, nothing happens by accident, and this wasn't an accident, either. The star performer's veteran staff had arranged places for the preferred questioners.

The theme of the new show, reassurance, came across most strongly when Reagan was asked whether he had anything to say to those "liberals and moderates who feel politically disenfranchised by your views."

No one has been politically disenfranchised, the president-elect replied.

Carefully pointing out that most of the states that had refused to ratify the Equal Rights Amendment, which he opposes, were governed by Democrats, Reagan sought to reassure those who think him insensitive to women's rights.

"Now, I am going to aggressively pursue the subject of equal rights for women," Reagan said, with appropriate seriousness. ". . . Those who chose to believe that my feeling about the amendment during this campaign meant opposition to equal rights to women were absolutely wrong."

And, finally, "President Reagan" was a patriotic movie in which the ex-actor playing the feature role solemnly reassured his nationwide audience that the entire nation wants its hostages back from Iran and that it is united behind President Carter to accomplish this.

It was a less exciting plot line, perhaps, than "Kings Row" or other movies the nation's new political star once made in Hollywood. And it was lots less warlike than the part this same actor had played in a series of fiery performances in the New Hampshire primary. But it was good theater. The extras played their roles. And the first performance of "President Reagan" was a success.