[verb] + 着 + [verb] + 着,(就) + [new event] + (了)
A one-syllable verb with 着, said twice, describes something going on for a while. The next clause says what happened in the middle of it – usually something unplanned. 就 and a final 了 often go with the second clause.
The second clause can be a change in the same person – an idea, a mood – or something from outside.
The pattern can't stand on its own. Something has to happen in the next clause.
It takes 着, which marks an ongoing action. 了 marks a finished one and doesn't fit.
Long verbs sound awkward in it. Keep to one syllable: 说, 走, 看, 聊, 想.