Board Thread:Manga/@comment-31209938-20170208033158/@comment-30069705-20170209050557

I don't know. But it's already proven that he needs to be in a certain state of mind or determined in order to transform, along with inflicting self-harm. I wouldn't be surprised if the same also needs to be true for him to activate the Progenitor Titan as well, except replace self-harm with making physical contact with a royal.

Either way, I really hope Eren at least talks this out with Historia in private. Otherwise, him keeping everything to himself could end up putting her in more danger, and not only her, but so many other people as well.