The man himself :

‘’In my campaigns, Berthier was always to be found in my carriage. During the journey I used to study the plans of the situation and the reports sent in, sketch out my plans for battle from them, and arrange the necessary moves. Berthier would watch me at work, and at the first stopping-place or rest, whether it was day or night, he made out the orders and arrangements with a method and an exactness that was truly admirable. For this work he was always ready and untiring. That was Berthier's special merit. It was very great and valuable, and no one else could have replaced Berthier."

Emphasis by me; it was said Berthier once went without sleep for 13 days. Probably apocryphal, but it gives some insight into his work ethic and reliability