General Brazilian
diálogo espontâneo
Most common usage across Brazil
Rio de Janeiro
papo natural / diálogo na lata
More colloquial; 'na lata' emphasizes authenticity
São Paulo
diálogo espontâneo / conversa de improviso
Standard usage with occasional use of 'improviso'
Portugal
diálogo espontâneo / conversa natural
Formal register preferred; 'conversa natural' also used