At dawn yesterday, Michael McMillan stopped moving. For the next half-hour, he didn't twitch a muscle. He didn't shift his weight, made no eye contact with anyone and had precisely as much facial animation as the nearby statues of Jefferson and Grant and Jackson.

To an onlooker he appeared to be a mannequin, a toy soldier blown up life-size. This is his job. It's a great honor. He was on guard, in the Rotunda, at the head of the casket of Ronald Reagan.

"A situation like this, you try to stay focused on a spot," McMillan, a 38-year-old Air Force master sergeant, said at the end of his 15-hour overnight shift. "You don't think about your body. You don't think about your aches and pains."

Ned Sullivan, a captain in the Marines just coming on duty, said, "You pick your point and you go to your happy place."

The body of the 40th president has been lying in state in the Capitol since Wednesday night, and at every moment an honor guard has been watching over him. The discipline of the guards adds a crucial element of solemnity to the room. Not moving at all becomes a feat of strength and endurance and service to the country.

Imagine the people who had gotten into the line on the Mall at 1:30 in the morning. They were blocks away from the Capitol, snaking through mazes, getting nowhere fast. By the time they made it up the hill and reached the west terrace of the Capitol, there was light in the eastern sky. Tired, hungry, bleary-eyed, they made a final march up a flight of steps and came to the doorway of the Rotunda. And suddenly all their chatter stopped. There was the casket, draped in a flag, almost glowing in the center of the softly lighted room. Three floral wreaths flanked the casket, and six guards.

An unarmed guard, such as McMillan, will always be at the head of the casket, and a guard with a rifle will stand at each corner. A sixth, the supernumerary, or "super," will take a position a few paces away. The rifles have no cartridges, no firing pins, but they have fixed bayonets.

Each "iteration" lasts 30 minutes. The changing of the guard is a stately, excruciatingly choreographed affair, with six new guards marching up two flights of steps. With representatives of each military service, the guards use what are called joint service movements, everything slowed down severely and synchronized. They don't really march: It's more like incrementalized gliding.

Each new guard takes a spot to the left of the old guard. All 12 salute. They present arms. There's a dead count, and then they order arms, dropping rifles to their sides. The old guards take two steps back, make a three-point turn and march-glide away.

The function of the "super" is to be prepared to take the place of any guard that falls. It's not a hypothetical situation. No guard is likely to pass out in the Rotunda, which is cool and pleasant compared with many of the assignments the guards pull. In brutal heat even the strongest soldier might wilt. Passing out is called "flakin'," McMillan said. Never happened to him, but he knows what it's like. Things just get real dark, and you're gone.

There are also no bugs in the Rotunda, another advantage over the assignments at Arlington National Cemetery.

"There are times when a spider will weave a web right on your hat, and a cicada will fly in your face," McMillan said. "You have to tough it out."

He's not making up the spider story. One crawled around his ear once, and he stood there, unflinching, telling himself that it would be okay if it crawled across his face but not if it tried to get into his ear. It finally vanished, perhaps crawling down his uniform. He couldn't be sure because he wasn't allowed to look.

Military honor guards handle parades, Pentagon arrivals, burials, sometimes several assignments in a single day, regardless of heat, rain, sleet or snow. The honor guards at the Tomb of the Unknowns did not leave their post even during Hurricane Isabel.

The Rotunda duty is by no means easy. The pressure is intense, simply because of the gravity of the event, the live television feed, the constant flow of onlookers. But the people with the toughest job are the body bearers.

Reagan's wood casket weighs about 735 pounds, says Capt. William Weber, the officer in charge of the Marine Corps "World Famous Body Bearers" (unofficial slogan: "The Last to Let You Down"). A normal casket might be 400 pounds or so. The body bearers have heard that Reagan's is lined with marble. Their job was more than simply lugging the casket from the caisson to the Rotunda, up the Capitol steps. They had to do so in a certain ceremonial way. No leaning. No visible struggling. Stone faces throughout.

It was so heavy that the first eight body bearers (who came from all five military services) had to switch out with another team halfway up the steps. Then the first team switched back at the top of the steps, carrying the casket into the Rotunda.

"You don't have the option of not being perfect," said Weber. "We can't afford to make mistakes. You have the pressure of the crowd, and you're carrying the casket, your muscles are tense, your body is hurting, and you're not allowed to make any facial expressions or show anyone that you're straining."

Weber wasn't on the casket this time. But six of his Marines were at various times this week. Cpl. Andrew Curtis, 21, from Baton Rouge, La., pulled that duty, serving on one of the leading corners of the casket. He felt honored to be part of the procession, and many times felt a chill run through him as he realized he was part of history.

But if he felt any strain, carrying the casket up the steps, he wasn't going to talk about it.

"We're trained to block it all out, sir," he said yesterday. "It could have weighed five pounds, it could have weighed 500 pounds, it doesn't matter. The job's going to get done."