"The president will do some surprising things at the General Assembly, some of which people will think are amazingly positive and some which people will think are amazingly negative", said Mr. Alterman.
"We know what his authentic voice sounds like and it is very different from what is scripted for him".
US President Trump will later today address the UN General Assembly, his first such address since entering the White House. The U.S. has asked member nations to sign a declaration on U.N. reforms, and more than 120 have done so.
The president is also expected to go after the United Nations itself, pushing for reforms that include more transparency and accountability.
"I think the main message is 'make the United Nations great.' Not again, 'make the United Nations great, '" Trump said as he left the U.N. building.
The annual gathering of world leaders will open amid serious concerns about Trump's priorities, including his policy of "America First", his support for the United Nations and a series of global crises.
Since taking office, Mr Trump has largely moderated his tone towards the United Nations, even inviting security council ambassadors to a working lunch at the White House in April. "That's a miraculous number".
Trump was also hosting a dinner for Latin American leaders.
Ahead of the opening of the General Assembly, UN member states will discuss the aftermath of Hurricane Irma that devastated parts of the U.S. and the Caribbean.
Aides said he will emphasize the need for states to promote peace and prosperity while upholding sovereignty and accountability as indispensable foundations of global order. The U.S has yet to make its payment this year, leading some in the U.N.to be fearful that it may slash its contribution.
Sen. McConnell says Suu Kyi urges aid for western Myanmar
The council on Wednesday issued a statement condemning "the initial attack on security forces and subsequent violence". It is hard to know her motivation; political calculation could well be at the core of Suu Kyi's silence .
"He will urge all states to come together to address grave dangers that threaten us all". And second, that he sees the United States as continuing to playing a key role in global affairs that goes beyond very narrowly defined national interests.
Later on Tuesday, Trump will have lunch with U.N. Secretary General Antonio Guterres, followed by meetings with Slovakia's Miroslav Lajčák, president of the U.N. General Assembly, and with the Emir of Qatar.
He is slated to meet with the Palestinians in the coming days.
The final day of the assembly Thursday will include meetings with leaders of Turkey, Afghanistan and Ukraine.
Mr Trump's speech is also unlikely to include any earth-shattering announcements of the kind made by the late former PLO leader Yasser Arafat in his 1974 speech when he extended an "olive branch" to the United Nations body.
In a show of strength today, the USA flew four F-35B stealth fighters and two B-1B bombers over the Korean peninsula, US ally South Korea said today, to "demonstrate the deterrence capability" of their alliance against North Korea's "nuclear and missile threats". The Obama administration prioritised multilateralism, however, working within the UN Security Council to achieve a resolution upholding the nuclear deal with Iran, though failing to make much headway through the body on Syria.
Less than a month after the country's sixth nuclear test - and its largest yet, at 250 kilotons - North Korea has said it intends to produce an intercontinental ballistic missile capable of carrying a nuclear warhead.
The president's visit to the UN comes amid talks between China and USA, with the White House saying the nations' leaders were "committed to maximizing pressure on North Korea" through sanctions. The United States is a veto-wielding council member, along with Britain, France, Russia and China.
When asked Friday what Moscow's message would be for Washington, Vasily Nebenzya, Russian ambassador to the United Nations, said, "Stay in the [nuclear deal]".