Board Thread:General Discussion/@comment-26456251-20160824191731/@comment-3152371-20160826013135

I think it's make sense to say that the player plays as Frisk's SOUL and share control with Chara's SOUL over Frisk's body during all the gameplay, in a similiar way that happened with Asriel. It make sense since the player control Frisk's SOUL, not Frisk himself/herself, during battles.

According Sans, "the more you kill, the easier it becomes to distance yourself". So the more  Frisk's SOUL (the player) kill monsters, the more the player distances from Frisk and gives room for Chara's SOUL take over Frisk.

It explains why the player totally loses control over Frisk during battle in the end of the Genocide Route - specifically when delivering the attacks that killed Sans, Asgore and Flowey. Also, it implicates that the place where Chara meets the player is in Frisk's inner self, where Chara kills the player - the jumpscare only occurs if the player tries to resist Chara's will - and take over Frisk entirely.