Until this week I thought I knew the customary things about Ronald Reagan -- his amiability, his informality (though I'm uncertain how this trait squares with the striped pants, white-tie-and-tails, mink coats atmosphere of his inaugural), his love for horses, his affection for the ranch, his union work, his record as governor of California (less clear here), his All-American-boy background before leaving the Midwest. And, in preparing for his inauguration, an event that appropriately faces West for the first time, I have been studying various authorities on him. Still, the picture of our soon-to-be 40th president remained fuzzy.

Now, thanks to Casey, Drake, George, Grover, Aimee, Charlotte, Helen and others who offer useful clues to the character of our new leader, much of the riddle becomes clear. The answer to the mystery of Ronnie lies, for all to see, on celluloid. He has been playing himself all these years.

In this jaded capital, Reagan's Hollywood career usually draws deprecating remarks. He was a Grade B actor at best, it is said, suited only to lowbrow stereotyped melodramas -- a performer incapable of serious films. So Washington says, in its superior way. I confess to have taken a whack in print myself at the "aging cowboy actor" turned politician. It's a bad rap.

After a week of watching Reagan's old movies, courtesy of local TV's Channel 20, it's easy to see why Ronald Reagan became a star, if not of the first magnitude, certainly deservedly popular. He's a more than competent performer, and comes over as likable, engaging and exuding a disarming charm. He was also, by all accounts, a serious, hard-working, dedicated craftsman.

When he played the part of Grover Cleveland Alexander, for instance, for three weeks before filming began he spent several hours a day working out with a leading major league infielder of the day. Since he was about to portray one of the greatest pitchers, he wanted to learn "the difference between throwing from the mound and just throwing," as he later told Tony Thomas, whose new book, The Films of Ronald Reagan , turns out to be enlightening.

Reagan displayed similar earnestness throughout his long career, which began in 1937 and some 53 movies later, ended in 1964. That was two years before he entered the statehouse in Sacramento.

He became a victim of success -- and typecasting. The Reagan films are all of a piece, classic Hollywood formula stuff; they always wind up the same way. Good guy gets girls, naturally.

Even when he plays second fiddle to a great romantic star and doesn't win the leading lady -- as when he played George Armstrong Custer to Errol Flynn's J.E.B. Stuart in "Santa Fe Trail" 41 years ago -- Reagan still triumphs. Though he loses Olivia de Havilland, Hollywood compensates by giving him the "right girl," Susan Peters. As the film ends, while Errol and Olivia are getting married in an ornate parlor car heading west, Ronnie and Miss Right are beaming at each other. Happiness reigns.

Seen from today's perspective, an unfair way of looking at them to be sure, the films are remarkably naive. They are also hopelessly dated. Their treatment of such things as race relations, women, Indians and other minorities is embarassing, if not outright offensive. But that's not what makes the Reagan films so memorable now. It's Reagan himself, the roles he plays and the values those movies embody that are illuminating.

In the Reagan films there's never a doubt about where the United States stands, never a question about the resolution of any problem facing the country. America is always right; no task remains beyond its capacity. Its people are brave, bold, good, generous. Its wars are just its land blessed, its progress assured. No matter how difficult the going of the moment, nor how terrible the ordeal to be faced, they will endure persevere and triumph. The vast majority of Americans portrayed are hard-working, salt-of-the-earth types, yet possessed of noble dreams. They are thrifty, rugged individualist whose best years, like those of their country, are before them. Optimism is their hallmark, simplicity their virtue. They know right will always prevail, honest effort will bear its own reward. Bad guys lose.

In short, they faithfully reflect the views most of us who grew up in the pre-Vietnam era held about the uniqueness of America, its destiny and special place in the world. And the character that Ronald Reagan always plays exemplifies those beliefs and qualities. He is the quintessential American.

The Reagan who flits across our screens in black and white, leading cavalry charges, threading his submarine through mine-infested Japanese waters, battling adversity and illness to stage baseball's greatest comback, and testing himself against innumerable hardships invariably does so with a self-assurance, a cockiness even, that shines through his various characters. There's nothing offensive about it, for he always tempers this trait with humor and a self-deprecating manner.

What's riveting, to this viewer at least, is how this celluloid Reagan matches the one we're seeing in real life taking command of Washington -- the confident stride, the quick grin, the easy wave, the breezy air of assurance. You have the eerie sense that you've seen it all before, that Reagan wasn't play acting at all during those long Hollywood years. He was only being himself. It's as if his entire life has been preparation to this greatest of all staring roles he's about to begin.

In his favorite role, that of George Gipp, the legendary and doomed Notre Dame football star, Reagan is ordered to carry the ball at his first practice. He picks up the football, cocks an eyebrow at Pat O'Brien, playing Knute Rockne, and confidently asks, "How far?" Now, 41 years later, for Ronald Reagan, once again life imitates art. As he picks up the ball this time, and asks the same question, the answer from a far greater throng remains: "All the way, Ronnie. All the way."