Talking to Himself: There is only one voice actor each for all male and female rebels. Any time two same-sex rebels have a conversation, it's with the same voice. Episode Two added a second voice actor for Sheckley since he was specifically written to converse with Griggs (who is voiced by the same guy as the other male rebels).
Done in-game during Point Insertion, as one man paces in front of the arrival/departure board repeating worries about the trains and disappearances. It's quite clear he's not talking to anybody but himself.
Throw It In: In the development of Episode 1, one of D0G's default animations played inadvertently during a scripted scene. It was so fitting that they hard-coded him to do it and wrote Alyx's dialog around it.
Alyx Vance: Well, Gordon... unless you have a better suggestion... He is a robot. He's done the math.