出于友情而来临时配合演出to perform as a favour to a friend(Britain); to perform as a favor to a friend(US)

• 友情出演的演员们，一为友情，二为添彩。

  Actors who perform as a favour to a friend do so out of friendship and also for prestige.

• 陆文夫、冯骥才等人都曾经“友情出演”过“名人讲故事”。

  People such as Lu Wenfu and Feng Jicai have all performed in "Stories Told by Famous People" as a favour to a friend.