Keep an eye on his fingers. And if he starts testily tapping his pencil on the table, back off.
That piece of advice for Serbian and Kosovar negotiators, who meet here today for a new round of talks on Kosovo's future, comes from belligerents in other conflicts who have settled their differences under the watchful - and sometimes exasperated - eye of Martti Ahtisaari.
The reputation of the self-deprecating former Finnish president as an impartial mediator has made him the world's "go-to guy" for international crises.
When NATO needed its surrender terms delivered to Slobodan Milosevic at the end of the Kosovo war, Mr. Ahtisaari was their man. He shepherded Namibia to independence, inspected secret IRA arms dumps as part of the Northern Ireland peace process, and last year brokered a peace agreement between Indonesia and Aceh separatists.
Now, as the UN Special Envoy for Kosovo, Ahtisaari is seeking an answer to one of Europe's thorniest questions: Can Serbs and ethnic Albanians agree on a status for the independence-minded Balkan province of Serbia-Montenegro?
Most diplomats would shy from that task, regarded by some as impossible. But as Ahtisaari said recently in a wide-ranging interview in his sparsely decorated office here, his track record gives him a head start. "I've been around and done so many things by now, it's easier to tolerate me than many others," he chuckled.
He brings another talent to the table, too. For a man who has spent his 40-year career as a Finnish Foreign Service officer and senior United Nations bureaucrat, he is unusually willing to stick his neck out.
"He is more a private-sector type than a typical administrator," says Juha Christensen, who worked closely with Ahtisaari last year while he mediated an end to the 30-year conflict in the Indonesian province of Aceh. "He has the ability to take risks."
Ahtisaari, who speaks fluent English in a quiet, measured voice with a slight Scandinavian rasp, was born with one natural advantage for a mediator: He comes from the almost obsessively neutral country of Finland.
More than that, however, he was born in Karelia, whose population fled en masse from a Soviet invasion in 1939 when Ahtisaari was two years old. An early childhood far from home, he says, taught him "sensitivity" to the plight of people caught up in wars. "I know what it's like when you are a refugee, living on the mercy of others, and having to adjust."
The experience also left him with a knack for feeling at home wherever he finds himself and a taste for impermanence - a taste he has indulged by working on four continents. The only house he owns, he says, is a summer place in Finland.
If he hadn't taken a job as a young man setting up a teacher-training college in Pakistan for a Swedish NGO, Ahtisaari says, he would probably have gone into local politics in Finland.
As it was, he launched himself on an international career that saw him move up the hierarchy of the Finnish government's Third World aid bureaucracy before being named the country's youngest ambassador, at the age of 36, to Tanzania.
There, responsible for handing out humanitarian assistance to liberation movements in Southern Africa, he earned the trust of African leaders who propelled him into his first internationally visible post in 1977, as UN Commissioner for Namibia - then an illegal South African colony.
It was in Namibia that Ahtisaari first cut his teeth as a mediator, though it would not be for another 13 years that the colony finally won its independence. He recalls how he brokered the first meeting between the South African authorities and leaders of the South West African Peoples' Organization (SWAPO), fighting for Namibian independence.
"We were all in New York," he remembers. "I told them that if they came to my apartment I would provide the electricity and take-out Chinese food and they could get on with it. I just went to my study. If you can make yourself useless and unnecessary in a process, that's the best thing you can do."
Ahtisaari showed his predilection for informal settings again last year when he chose a small mansion outside Helsinki as the site for a series of negotiating sessions he chaired between the Indonesian government and leaders of the Free Aceh Movement (GAM), which ended GAM's guerrilla war of independence with an agreement on autonomy.
His presence, however, proved necessary right up until the last moment, cajoling, probing, reassuring, and occasionally threatening the two sides.
By then, he had the weight of experience behind him, having played key roles in Namibia's independence and the Northern Ireland peace process, among other troubleshooting assignments.
He'd also completed a six-year stint as president of Finland.
Though GAM leaders were often disappointed in him, says GAM negotiator Bakhtiar Abdullah, "we respected his past achievements in other fields - and the fact that he was a past president - he was a credible negotiator."
His style as a mediator, say people who have worked with him, is firm. "He knows how to play stern, how to be like a father," says Sofyan Jalil, the Indonesian Minister of Information and Communication, who was on the government negotiating team at the Aceh talks. Mr. Christensen has another way of putting it: Ahtisaari is like "a headmaster," he says.
"There is no point in being nice for the sake of being nice," explains the man himself. "You can't avoid difficult issues."
Underlying this approach, says Sidney Jones, an expert on Indonesia with the International Crisis Group who briefed Ahtisaari before the negotiations began, is enormous self-confidence.
"He wasn't particularly knowledgeable about Aceh," she recalls, "but he had no doubts that whatever the difficulties, he would get over them."
Ahtisaari doesn't quarrel with that. "In this business you have to have fairly strong self-confidence," he says. "But you mustn't use it as an ego trip."
Others see his self-assuredness in a less flattering light.
"Personally I don't like him because I feel he's very, very arrogant," complains Mr. Abdullah. "He doesn't seem to have a sense of humor; he is more of a very firm person."
Generally, say people who know him, Ahtisaari is a good listener. "He didn't lead from the front, saying 'this is how we are going to do it,' " recalls Damien Kingsbury, an Australian academic who advised the GAM delegation at the Helsinki talks. "He wasn't a one-man show, and he listened to the other parties."
Sometimes, though, he can flare up. "It's easy to see when he's mad," says Mr. Bakhtiar. "He listens to you attentively with a sour expression, then he just bursts and throws his pencil on the table. Then, you know, uh-oh."
Ahtisaari smiles when asked about his temper tantrums. "Perhaps a little bit of theatrics help in some situations," he acknowledges. "I can probe them, nudge them forward, sometimes be fairly tough. But I'm an old man. I tell my younger colleagues 'Don't you try this.' "
As an "old man" past normal retirement age and entitled to a generous UN pension, why does he go on doing what he does?
"Perhaps it's a curse, but when you are a Lutheran you have a sense of responsibility," he says with a shrug.
"And I'm one of those people who as long as I am still healthy and my thoughts are more or less clear, I don't think I can retire entirely."
At the same time, he adds, "I am in this game for root causes. I am very practically oriented and I want to have results. These tasks are such...that you can put a little bit extra into the efforts of extraordinary people to help them" succeed.
"But I have no superhuman powers," he says. "If people don't want to make peace, there is nothing I can do."
1977-1990: Led the UN efforts to win Namibian independence from South African rule, brokering the final face-to-face negotiations.
1991: Led the UN mission immediately after the Gulf War to evaluate the humanitarian and economic situation in Iraq.
1992-1993: Engaged in the failed international efforts to end the war in the former Yugoslavia
1994-2000: President of Finland
1999: As EU special envoy on Kosovo, delivered NATO's surrender terms to Yugoslav President Slobodan Milosevic.
2000-2001: Inspected secret IRA arms dumps to ensure weapons out of use.
2005: Brokered a peace agreement between the Indonesian government and rebel forces in the province of Aceh.
2005 - present: Chairing UN-sponsored talks between Serb and Kosovar authorities on the future status of Kosovo.