Casey's pretty much playing escort. He's used to shadowing people and offering tactical support anyway. So, he accompanies Guinevere, her guard and companion, because he knows Morgana would want someone making sure she's kept safe. He doesn't talk much, just walks at her side and glances over at her occasionally to see how she's holding up.

He pauses and checks out their surroundings while Guinevere makes the call. Once she's done, he turns back to her.

"You think he's gonna let us see her? She's gotta be behind one of these doors."

Just then the Doctor's voice can be heard, so Casey shuts up and lets them talk.