"It’s like the meeting with Putin is the same thing as him waking up at six in the morning and tweeting," she commented, noting Mr Trump's affinity to use Twitter to communicate with the public about everything from his distaste for the "fake media" to foreign policy matters.
Mr Putin, a former KGB spy, will come with his own agenda, said Ms Liasson.
What may end up being just a so-called "elephant in the room" are the ongoing investigations by the FBI, Senate, House, and special prosecutor Robert Mueller into alleged ties between Russia and members of the Trump campaign team.
There is no indication Mr Trump will discuss that or the January 2017 report which intelligence agencies signed saying that Russia tampered with the 2016 US election.
During a stop in Poland before heading onward to Germany, Mr Trump said: "I think it was Russia and I think it could have been other people and other countries. It could have been [that] a lot of people interfered."
He did not name any other countries that could have possibly hacked the US election, however.
" Being unpredictable is not necessarily bad, being unprepared is a different matter," said Ms Liasson. She described Mr Putin as "cagey" and someone who would do a "deep dive" into Mr Trump's psyche, which is clearly on display through Twitter.
North Korea is another topic that will be on the larger G20 agenda.
Ms Liasson said: "North Korea sees nuclear weapons as an insurance policy".
With China and Russia both asking the US to not place new sanctions or use military force to stem the threat from Pyongyang. China's concern is also regional and peninsular security.
Mr Trump has expressed some disappointment with Chinese President Xi Jinping regarding the situation.
The mercurial nature of Mr Trump and the strained relationships he already has with German Chancellor Angela Merkel and French President Emmanuel Macron, may be a worry for Korea's President Moon Jae-in.
He recently visited Washington and it was clear Seoul does not want the US to "take any kind of precipitous action" with Kim Jong-un because of its close proximity to the North Korean border and non-nuclear missiles, according to Ms Liasson.