According to Oscar Isaacs, his first scene was the "torture" scene. Adam Driver never took off his helmet but would lean down to whisper "it's okay, it's me in here" between takes.
Which leads me to really want Ben undercover in the First Order. He and Poe had some sort of relationship (platonic or romantic), and when Poe's captured Ben still has to do his job. But he still wants to take care of Poe, even though it really will hurt to extract the knowledge and Poe won't give it up on his own. So Ben has to be scary and intimidating while still caring for his friend.
Poe/Kylo; undercover!Ben; good aftercare behavior
Which leads me to really want Ben undercover in the First Order. He and Poe had some sort of relationship (platonic or romantic), and when Poe's captured Ben still has to do his job. But he still wants to take care of Poe, even though it really will hurt to extract the knowledge and Poe won't give it up on his own. So Ben has to be scary and intimidating while still caring for his friend.