At some point in a story a character will realise that he’s got to do what he’s got to do. There’s no turning back.
This can happen at any time. On the first page, just before the climax, or anywhere in between—it doesn’t really matter as long as it makes sense within the story.The important thing is for the reader to see this moment so they understand how the character feels and why.
It isn’t enough to just assume the character’s reasons will be taken for granted or accepted without question.