When he passes by, I will pretend wanting to take picture(selca) together with him, and when he stands still, you have to grab his body quickly! => When he passes by (그 사람이 지나갈 떄), I will pretend ... with him (내가 함꼐 셀카 찍고 싶은 척할 테니까), and when he stands still (섰을 떄), you have to ... => 그 사람이 지나갈 때 내가 함꼐 셀카 찍고 싶은 척할 테니까, 섰을 떄 재빨리 그 사람 몸을 붙잡아.