Full Idea
Character is constant over a period of time; the way a person is at a single moment does not at all reflect the character of this person. We do, however, concede that character changes.
Gist of Idea
We don't see character in a single moment, but only over a period of time
Source
Simone Weil (On the Concept of Character [1941], p.98)
Book Reference
Weil,Simone: 'Late Philosophical Writings' [Notre Dame 2015], p.98
A Reaction
I do think, though, that there are moments in behaviour which are hugely revealing of character, even in a single remark. But I agree that most single moments do not show much.