The Important Witness (1933)

A public stenographer, taking private dictation, sees a gangster shot in his room and she is taken along by the killers. The gang-chief falls for her, and she plays along to lull his suspicions, and eventually makes an escape. She takes a night bus back to Los Angeles in order to give testimony that will clear her girlfriend who has been falsely accused of the killing. But the gang pursues her and she must take over driving the bus when the driver is shot.