What were you doing when I called last night? I was sitting in the café when you called. Saule was watching a film when she heard the noise. Yesterday I went to the library. We were playing tennis when Serik hurt his ankle. Omar was taking a shower when the telephone rang.
past simple используем для описания коротких действий в прошлом (called, went, heard)
past continuous для более длительных действий в прошлом (was watching, were doing)