London — About to sit down with Walter Mondale, George Shultz, and Ronald Reagan is a dour, watchful, thin-lipped intellectual who doesn't smoke, who rarely if ever drinks, and who once dismissed a US reporter's questions about his life with the remark, ''My personality does not interest me.''
He seldom relaxes, but when he does he either plays chess with his wife or takes off on long walks at such a rapid pace that he tends to leave his companions behind.
Andrei Andreyevich Gromyko is, in fact, one of the most remarkable and durable diplomatic phenomena of this century.
He has been in the Soviet Foreign Ministry for 45 years. Promoted rapidly to fill posts vacated by Stalin's purges, he headed the American desk just after he turned 30.
He was ambassador to the United States at 34, and, still in his mid-30s, attended the Tehran, Yalta, and Potsdam conferences, and took part in the Dumbarton Oaks and San Francisco conferences that established the United Nations. And he shook hands with his 11th US president on Sunday.
When Ronald Reagan was making movies, Gromyko was making headlines as Soviet ambassador to the UN, where he has cast 26 vetoes in the Security Council, earning the nickname ''Old Stone Face.''
Here is a man who spans Soviet leaders from Stalin to Chernenko, who has been foreign minister for 27 years, who has done business with 14 US secretaries of state.
Gromyko has risen from the status of a foreign affairs functionary to a senior Politburo figure now at the peak of personal influence as the voice of foreign affairs under a leader unwell and unversed in foreign ways.
Gromyko is comparable in longevity in office only to two much more flamboyant diplomatic legends of the 19th century: Metternich and Talleyrand.
Perhaps the aptest comparison is with Prince Talleyrand, who lasted through the French Revolution to continue to serve the restored monarchy of the Bourbons.
For Gromyko is the classic survivor.
He spoke for Stalin to Franklin Roosevelt. He cast the famous vetoes, stood implacable over Berlin, denied that there were any Soviet missiles in Cuba ... and then negotiated the two symbols of Brezhnevian detente with the US, the SALT I and SALT II arms control treaties.
Since then, he has reverted to confrontation as Poland, Afghanistan, human rights, NATO's cruise and Pershing missiles in Europe, Reagan policies, and Kremlin leadership changes have all worked to encase detente in blocks of diplomatic ice.
Does all this mean that Gromyko is a man without views of his own? How can he chop and change so radically over the years? And what are the qualities that have caused US officials from Dean Acheson to Cyrus Vance to praise his diplomatic skills?
''Basically, Gromyko is a cautious man,'' says a US diplomat with considerable experience at close-up Gromyko-watching.
''He works in a system and in a society where there are definite penalties for taking risks and failing.
''Yes, he has served five Kremlin leaders, but to a Soviet official, duty lies in representing the leadership line of the moment.
''A man like Gromyko would not feel bad about serving his country that way. He'd feel good about it....''
Former US Ambassador to Moscow Malcolm Toon said in a telephone interview from Washington: ''Gromyko has been able to maneuver within a very tough society. He's flexible.
''But he's no cipher, certainly not now. He had little power in his early days. Khrushchev once said, as Gromyko listened, that Gromyko would sit on a block of ice until ordered not to. But since Brezhnev brought him into the Politburo in 1973, you haven't heard a derogatory word about him.''
Brezhnev found Gromyko's experience ''terribly useful'' when dealing with leaders such as Richard Nixon, Ambassador Toon says.
To Toon, a veteran of many encounters with Gromyko, the foreign minister has come to symbolize stability and continuity in foreign policy at a time of change and ill health among the last three Politburo leaders.
''Gromyko now has a powerful role in the formulation as well as the implementation of policy,'' he says.
A clue to Gromyko's beliefs comes from former State Department official Helmut Sonnenfeldt, who once said Gromyko greatly admired the man who held his job for 26 years under the czars - Prince Alexander Mikhailovich Gorchakov, foreign minister from 1856 to 1882.
Gorchakov helped rebuild Soviet influence abroad after the defeat of the Crimean War. Gromyko, as a fierce patriot, wants to expand the influence and prestige of the Soviet Union in whatever way the collective leadership decides is expedient.
There is, as might be expected in a man who rose to prominence under Stalin, an element of ruthlessness in the Gromyko character.
Many Soviet-watchers think Gromyko is by nature a hard-liner, happier glaring at the US than smiling at it.
''By nature, Gromyko is a Stalinist,'' says Old Bolshevik Iosif Pavlovich Itskov, who emigrated from the USSR in 1968 after the invasion of Czechoslovakia and now lives in West Berlin.
Interviewed by telephone after he had broadcast 17 15-minute programs on the BBC Russian Service about a party career spanning six decades, Mr. Itskov predicted no Gromyko smiles at the meeting with Reagan.
''Look,'' he said, ''the Kremlin failed to persuade European public opinion to reject NATO's cruise and Pershing missiles.
''The Politburo has concluded that Reagan is about to be reelected for four more years.
''These men are pragmatists, opportunists. They want to see whether some kind of dealing is possible with a man who is going to be president for a while yet....''
The US diplomat who studies the Kremlin agreed.
''This collective leadership knows the USSR has severe problems, and that they don't know the answers,'' he said.
''It is looking for a breathing space. Of course, it will embarrass us if it can, but Gromyko's task is to find out if any kind of movement is possible. That suits his personal style, too.''
Says former Ambassador Toon: ''The Soviets are not fools. They don't want to alienate permanently the President for the next four years.
''Their isolation from and hostility toward Western Europe hasn't paid off, and they want some kind of arms control deal for economic reasons: They need more money for their own economy....
''I don't see any great breakthrough here. I don't see anything substantive coming out of the Reagan meeting. But it's a start. It's a good thing to reinstitute dialogue.''
What has put Gromyko at the center of such superpower talks for almost five decades?
''He has a fabulous memory,'' says the US diplomat. ''A Soviet friend told me he once met Gromyko in a certain spot in a certain room, then didn't meet him again for several years. When he did, Gromyko recalled where they'd met, the actual spot, the room, what they'd talked about ... then apologized for not remembering my friend's name.''
Gromyko rarely works from notes. His recall is almost total. He does his homework. Nor can anyone match his sheer experience. He can say about every single important East-West meeting in the last four decades, ''Yes, I was there.''
Significantly, he is loyal to the party, while lacking the kind of party political base that might make him a contender for the post of party (and thus Soviet) leader. He is not seen as a threat by Chernenko, it is believed, but as an indispensable foreign policy asset who knows foreign leaders from repeated personal meetings.
In a leadership that rarely travels to the West at all, that is a priceless asset.