Julian Fellowes, Gareth Neame and Liz Trubridge:
SE02 EP01 Commentary.

Gareth: I think, Rob had a huge amount of story to tell this episode. It was really, a massive journey to go on, in these few scenes, to say that he (Thomas) has so completely lost it… because of the trauma. You know, the trauma of this war situation that he’s prepared to…

Liz: Do this. 

Gareth. …I think he does it really well.

Liz: I think he’s brilliant with it. He (Rob) was very affected by going in those trenches…