By the time a man reaches the age of 69, a large portion of his predictability can be discovered by studying those experiences that were important to him.

If you're an actor, you get up in the middle of the night to go to work. Your place of business is a set designed to look real. You get into a costume, people bring your coffee, you're made up. A crew in charge of cameras, lighting, scripts and other details moves about. You don't question what they're doing. Someone explains today's scene. You perform. Then you do the same thing over and over again until the director is satisfied. Critics ultimately review the picture. You become used to receiving the credit or taking the blame for a product that was not wholly yours.

Presidents are writers, directors, producers and actors rolled into one. They're the whole show. They take advice but only at their own risk. It's their own neck, nobody else's. Just as an actor can't blame a poor performance on poor direction, a president can't blame his foreign policy on the secretary of state. There's no way to hide; the buck stops there.

Reagan is comfortable with the essential responsibility of the presidency. He is prepared by the discipline of his former profession to let the critics judge his performance. But can he adjust to a situation where there are no retakes, where others will be looking to him to describe the scene? And can he also play the role that is now demanded of him? The first substantial test of this will come tomorrow when he announces his selection of a vice president.

I have been asked many times how Reagan goes about making a decision. The answer is that his decisions rarely originate with him. He is an endorser. It is fair to say that on some occasions he is presented with options and selects one, but it is also true that in other instances he simply looks to someone to tell him what to do.

It is this endorsing process that accounts for the difference between Reagan the campaigner and Reagan's more moderate record as governor of California. The white-carded stump speeches are Reagan the performer playing to a known audience and sending the crowd away with its money's worth. As governor, there was no crowd, merely decisions to be made, only a few of which were very exciting. Reagan sat with his California Cabinet more as an equal than as its leader. Once consensus was derived or conflict resolved, he emerged as the spokesman, as the performer.

It didn't bother him that many decisions reached during his governorship were in severe conflict with his campaign oratory. While he was running for governor, one of his pledges was to hold the line on states taxes; one of his first acts as governor was to raise taxes. Reagan sees no conflict in this; it simply had to be done. His advisers had no option that would allow the pledges to be kept.

I would point out that there are indeed limits to the advice that Reagan will accept. Had his director in "Bedtime for Bonzo" demanded that he play a pivotal scene in the nude, he would have refused. If nuclear war were suggested as one option to President Reagan, he'd pick something short of it. If his advisers are adequate, there is nothing to fear from President Reagan.

But he can be guided, and presidents who are too easily guided run the risk of losing the confidence of the people. This is one of President Carter's problems: people don't think he's in charge.

Indeed, this is the difference between being governor of California and being president. It's not just that the job is bigger; it's more that you must dominate it. You don't come to terms with the presidency; you grab it by the neck and you never let go. You must ride the tiger or you'll wind up on the inside. You can't be civil with the presidency; it doesn't fight fairly.

I have been amused this past week to read the newspaper accounts depicting Reagan pondering over his choice of a vice president. Reagan hasn't been losing any sleep over the vice presidency; he's been off somewhere working on his acceptance speech. Such are his priorities, developed long before he entered politics. The audience must first get its money's worth.

Sometime tonight, the options on the vice presidency were finally reviewed. It won't matter if Reagan's oft-stated campaign pledge to pick a vice president who fully agrees with him on the issues may have been breached. Reagan will endorse the collective wisdom.