observer

一位中立的观察员被请来评估局势。

A neutral ___observer was brought in to assess the situation.

在这项心理学研究中，她扮演被动观察者的角色，只做记录而不进行干预。

During the psychological study, she acted as a passive ___observer, taking notes without intervening.

作为旁观者，他注意到了会议上的紧张气氛。

As an ___observer, he noticed the tension during the meeting.