The airport scene has been discussed in numerous articles, and Fischer has given the answer herself:
During an Instagram live on Tuesday, May 1, Fischer — who still has an homage to Pam in her Instagram bio — finally revealed what she and Steve Carrell were talking about in that famous scene. As it turns out, Fischer felt just as strongly about Carrell's departure as Pam did about Michael's, because the emotions fans see on screen were totally real:
That was me talking to Steve. I told him all the ways I was going to miss him when he left our show. Those were real tears and a real goodbye. That was a really emotional scene.
In her talking head interview after she and Michael have parted ways, Pam is crying, so I have to imagine that these were real tears, as well. "No, he wasn't sad," she says through tears. "He was full of hope. About Colorado. And he was hoping to get an upgrade as an awards member. And he said that he was just real excited to get home and see Holly."