The definition emphasizes the capacity for expression. Someone may possess that capacity while silent, unwilling to speak, or not presently engaged in conversation.
A teacher's linguistic competence does not disappear whenever the teacher falls silent. Conversely, a person can speak continuously while producing defective constructions; volume of speech does not itself establish speaker-fasaha.
Two mistakes should be avoided: silence does not prove incapacity, and frequent speech does not conclusively prove fluency.