«إِذْ وَقَفَ بِنَا شَخْصٌ»
Suddenly, a person stood before us.
Here idh marks a change in scene. After the established rhythm of literary conversation, the short verb waqafa abruptly introduces an unexpected participant.
The narrator does not yet name the visitor. That delay creates suspense: who has arrived, what does he want, and how will he address this gathering? The next phrase supplies the first clues through clothing and movement.