Among familiar friends, she finally opened up and became talkative.

opened up 是动词短语 open up 的过去式形式。open 本义是"打开"，up 表示"完全、彻底"。 open up 作为短语动词，意思是"敞开心扉""放松地交谈"，指从封闭状态转变为愿意分享和表达的状态。