Although he is surrounded by more authoritative experts than any chief executive in the world, every American president seems to feel the need for an additional, informal adviser in or out of the government -- a close friend and confidant who thinks the way he does, someone he can turn to for dispassionate counseling.

For President-elect Reagan, that man is William French Smith.

The affable, balding, round-faced Smith has been Reagan's personal lawyer, dinner companion and intimate associate for many years. Their friendship goes back to the days when the president-elect was head of the Screen Actors Guild.

Those close to the two say it was Smith who originally encouraged Reagan to get involved in politics. And since Reagan took the plunge 16 years ago, it has been Smith, as much as anyone, who has guided and nurtured Reagan's astonishing rise from a fading Hollywood actor to president-elect of the United States.

Wherever he is, Smith will have the new president's ear more than anyone -- with the possible exception of Nancy Reagan, who, despite her humble insistence that she will play only a wifely supporting role, probably has as much influence with her husband as Rosalynn Carter has with hers.

Smith was one of the first people Reagan consulted when he was considering his last try for the White House, and he was the first one he turned to when he won the long-sought prize. At Reagan's request, Smith converted his office in the Los Angeles law firm of Gibson, Dunn and Crutcher into an informal transition headquarters two months before the election.

From this unlikely command post, Smith handpicked the top members of Reagan's transition team, personally screening the handful of Republicans and Democrats who have in turn chosen the key officials for the new administration.

Since the landslide victory, Smith has sat at the head of Reagan's informal "Kitchen Cabinet," the small group of conservative businessmen and financiers, like Justin Dart and Holmes Tuttle, who for years have been the new president's most determined, loyal and influential backers.

What kind of man is Smith? For one thing, he is a man who keeps a low profile: he didn't even make "Who's Who?" until the current edition, which went to press last May.

Like Reagan, Smith is not a native Californian, or even a westerner. He was born in New Hampshire and moved to California as a young man. He graduated from the University of California, where he made Phi Betta Kappa, and got his legal education at Harvard Law School. He served in the Navy during World War II, reaching the rank of lieutenant.

Smith is a rock-ribbed conservative. An engaging man with a quiet sense of humor, he is not averse to the simplistic one-liners that have characterized the Reagan campaigns. One of his favorite homilies probably sums up his political philosophy while it needles his political opponents: "It takes liberals years to learn what a conservative can do by immediate instinct."

Smith's pragmatism was demonstrated during Reagan's two terms as governor, when he helped his boss get along with a Democratic state legislature. He was the governor's alter ego on the state universities' board of regents, where he worked quietly but effectively to push the governor's conservative policies.

Smith's political flexibility is also shown by his close personal and professional relationship with Los Angeles' Democratic mayor, Tom Bradley, whom Smith has advised on such matters as labor relations and the 1984 Olympics bid. In similar practical fashion, Smith lent a bipartisan tone to the Reagan transition team by bringing onto it Democratic Sens. Henry Jackson of Washington and Richard Stone of Florida.

Smith told my associate Bill Gruver the transition has not been as difficult as he and other Reagan advisers thought it would be. "We have tried to put together a highly professional administration to give people a feeling their government is being run competently," Smith said.

As for the incoming president himself, Smith characterizes his old friend as "a good user of people's abilities," and adds, "The government doesn't overwhelm or overawe him."

Smith compares the prospective Reagan Administration with that of John F. Kennedy. When Kennedy took office in 1961, Smith explains, "he excited the American people, giving them a feeling of prestige and self-confidence." Smith thinks this is what will happen when Ronald Reagan moves into the White House.