Here they come, walking around the room, satins ruffling and smiles fixed. Egad! She's caught your eye. Good Grief! He's heading toward you. You open your arms to give a welcoming bear hug, and suddenly -- Oh No! -- you remember the First Royal Rule.

"They are very definitely not to be touched -- other than the shaking of hands," strictly advises Anthony Burns, press attache' for the British Embassy. "Grabbing them by the arm would certainly not be well viewed."

Confronting Charles and Diana, of course, is something only a minuscule number of Washingtonians will have to do, but it could be the most cataclysmic event that ever happens to them. Socially speaking.

And what may be most perplexing to outgoing, glad-handing Americans is that Their Royal Highnesses Charles and Diana glide through life in a sort of unapproachable state, somewhere between limbo and nirvana.

Only they may approach; they may never be approached. Only they may address; they may never be addressed. Only they may ask; they may never be asked.

To drive home the fine points of protocol, the British Embassy has been thoughtful enough to phone up at least one expected guest.

Robert Alcorn from Global Travel has worked with British Heritage, an English tourism group, and so was extended an invitation to Sunday's dinner at the embassy. Unfortunately, he can't go because his weekend home has been flooded. Nonetheless, the embassy advised.

"A woman from the embassy called to give me instructions on what to do," he said. "She said I have to stay in the background and not go near them. Basically, just to wait. She said there would be no receiving line and if Their Highnesses wanted to talk to me, they would walk up."

There's more. Burns speaks sternly about the protocol of addressing them: "The right way -- the British way -- is 'Your Royal Highness,' and thereafter to refer to her as Ma'am and him as Sir." "As always, the secret to good behavior is good behavior -- being polite and courteous, not intrusive or overfamiliar. You don't say, 'Hello, Di,' or 'Hello, Princess.' "

Of course, there's the ever-controversial question of the curtsy. Whatever the British say, the American rule is not to, as former protocol chief Leonore Annenberg learned to her chagrin the last time Charles was here.

Okay, so you've avoided the most obvious faux pas. Now, the hard part. What do you say to a real live Royal Highness? What the Pros Advise:

Walter Annenberg, former ambassador to the Court of St. James's: "Well, what you don't ask Charles about is the royal family. Ask him about his mother, he'll look at you like you have two heads."

The Honorable Simon Howard, a lender to the National Gallery exhibition: "Goodness, I don't know. I've never been in that situation. I suppose art."

Bonnie Swearingen, Chicago socialite: "Well, I have met Charles at a dinner in London and I was wearing the most beautiful dress. It had a cascade of flowers on the shoulder and he thought they were real. He said my dress was the loveliest one there, and I told him the flowers weren't real. He said it didn't matter, it was so lovely. I was just captivated . . . So if I were to see him today, I would say 'Why don't you and I go on a private picnic together?' "

Peter Duchin, band leader: "What are you doing around hunting season? Hope we can shoot some birds this year."

Malcolm Baldrige, rodeo fan: "Hello. That's all, just hello."

Jerome Zipkin, man about town: "I think they're young enough and with it enough that one could talk about almost anything. I would talk about New York, Washington, London and points east and west. That should do it." Of course, even with all these marvelous suggestions for scintillating conversation, there are a few taboos. Consider the following: What Not to Say to Charles:

"Now, your great-grandfather, was he the one with all the mistresses?"

"I just loved that book by your valet."

"Say, would you two like to grab a quick drink after this?"

"I use a Ouija board, too!"

"So, when do you think you'll be king?"

"Hey Chuck, you have something on your chin."

"How's your brother Randy . . . I mean Andy?"

"Do you mind not interrupting?"

"Your wife is so tall."

"I can't stand my father either."

"I'm going to be in London in a few weeks. How do I get in touch with you?"

"We keeping you awake?" What Not to Say to Diana:

"Do you ever use that bowl the Reagans gave you?"

"Do you all still see Koo Stark much?"

"May I call you Diana?"

"Have you lost weight?"

"Have you gained weight?"

"Are you pregnant?"

"Oh, I was just rereading 'On Being and Nothingness.' "

"I have that same dress."

"I've been called a princess too."

"Can I try on your ring?"

"How are you getting along with your mother-in-law these days?"

"May I have a taste of your salad?"

"Good help is hard to find, isn't it?"

"You know I paid $50,000 to be here tonight."

"Oops, so sorry . . . Promise you'll send me the cleaning bill."

"I'd take it easy on the sherry if I were you."

"I'm sorry, I don't understand a word you're saying."

"May I cut in?"