It is a political play with a peculiar twist. The leading player doesn't enter until the denouement, but the stage is set months before curtain time. The inaugural ceremony, the biggest show in town, comes to Washington Jan. 21, and hundreds of people have been scurrying behind the scenes since April.

One official stroke in the process occurred on the west side of the Capitol yesterday morning, as Sen. Charles Mathias (R-Md.), chairman of the Joint Congressional Committee on Inaugural Ceremonies, drove the first 16-penny nail into what will be, by December, a $239,000 inaugural stand.

As of yesterday, said an inaugural committee spokesman, construction trailers had arrived, plywood was stacked on the site and workers were painting primer coats on the 2-by-4s.

But building the platform is only a part of the inaugural preparations, which involve the Pentagon, Metro officials, the FBI, the National Park Service, District Police and others.

They encompass everything from designing the parade's route to making box lunches for participants.

"There are a lot of players, and it becomes very complex," said U.S. Navy Cmdr. Greg Gagne, spokesman for the Armed Forces Inaugural Committee. "There are so many agencies that have a piece of the action."

Gagne said the AFIC preparations began with a staff of five or six people and have now grown to include more than 200. The committee's budget for the inauguration, which will involve nearly 12,000 military personnel "in some fashion or another," is $539,000, "and that's just us," Gagne said.

The planners said this inauguration, the nation's 50th, will be much like its predecessors, despite the fact that the only required element is the constitutionally mandated swearing-in. The date is Jan. 21 because the traditional date, Jan. 20, is a Sunday.

Gagne said his committee members began preparations several months earlier than usual this year so they could take careful notes as they worked, leaving a script for future planners to follow. "Up until now, there has not been a comprehensive plan left behind -- no cookbook, as it were," he said.

Despite the most concerted efforts of scores of planners, some decisions must be left until the last minute -- or at least until Nov. 7.

Some past presidents-elect have ordered ceremonies designed to their specifications, such as Harry Truman's request for a B-52 flyover. "President Carter wanted to walk down Pennsylvania Avenue. That was kind of different. The Secret Service had apoplexy," Gagne said.

A member of the inaugural committee said he had heard no special requests from either Walter Mondale's or Ronald Reagan's headquarters. "I truly suspect they both feel it's bad luck," he said. CAPTION: Picture, Construction workers begin building a $239,000 presidential innaugural stand at the west front of the Capital. By Harry Naltchayan--The Washington Post