to give someone extra or missing information

I filled her in on the latest gossip. 我告诉了她最新的小道消息。

to do someone else's work for them because they cannot or will not do it themselves

She will fill in for him while he's at the conference. 她会在他开会时代理他的工作。

I'm not her regular secretary - I'm just filling in. 我不是她的常任秘书，我现在只是临时顶替。