Mongke walked out of the planning meeting, his mind already wandering somewhere. When he returned to his quarters, Mongke summoned a pen and parchment and started writing. Despite how small the brush was, Mongke's writing was impeccable due to his ungodly control over his body.
He wrote notes or observations about Wang Wei, from the latter's behavior, mindset, tactical abilities, and even subtle micro-expressions. Mongke even wrote about Wang Wei's flaws, like his often casual demeanor, which Mongke viewed as not appropriate for a leader. If your subordinates are too comfortable for you, it becomes easier for them to walk out of line.
Of course, Mongke also recognized that Wang Wei can instantly switch from joyous and casual to noble and intimidating. His personality could not walk this fine line, so Mongke decided not to learn from this aspect. Another critique he had was that Wang Wei was not ruthless enough.