Full Idea
As a botanist can recognise the whole plant from one leaf, …so an accurate knowledge of a man's character can be arrived at from a single characteristic action.
Gist of Idea
A man's character can be learned from a single characteristic action
Source
Arthur Schopenhauer (Parerga and Paralipomena [1851], VIII:118)
Book Reference
Schopenhauer,Arthur: 'Essays and Aphorisms [from Pand P]', ed/tr. Hollingdale,R.J. [Penguin 1970], p.144
A Reaction
Very true. Great novelists specialise in such observations. One word can reveal a character, as well as one action.