I am not referring to the mortal arch. I believe DH is intrigued by FJ since the moment he spies her talking about him to SM . That's why he plays a trick on her at the wedding. And he finds her beautiful. He says so to Lian Song; he says that those who called FJ a beauty had good taste. But when does he fall for her?

I think that he starts to fall in love with her when FJ helps him fight the witch in Aranya's dream. Like he said, it's the first time in his life that someone seeks to protect him, instead of seeing his protection. 

What do you think?

What about FJ? She loved him all along, but at first, it was the kind of love a teenager girl has for her idol. When do her feelings become more mature and real?

From the novel, he interested in her the moment he saw her at the party when she dropped the flower pot and blamed it on Migu. But I guess in the drama, it was a bit different. From what I understand, he liked her the moment he saw her. 

I think he loves her when he puts all the parts together- her being the fox.  And before Aranya's dream as he finds away to even show up in the bird kingdom. It was suppose to be another that was their teacher that time. He dose go there but it was early. He went just to be with her. You have to remember that she protected him as a fox already.

Hers... is around the time she gives her shadow/mortal trail. 

I think he liked her the moment he saw her.  I think because of his lack of insight into emotions he couldn’t put it together.  If you read the novel, his behavior all points to him liking her (from the first chapter. Remember, the show is told in a linear way but the book takes place after the immortal arc and the stories of the fox etc. are told through flashbacks that FJ has when she is triggered or feeling sad) .  DH cannot make heads or tails of his interest in her so he behaves immaturely, finding ways to keep her around or stay near her, unsure why he is fascinated by her and unsure why he cares about her safety or why he dislikes Yan Chiwu.  since he is neither good nor evil and is shameless, and a super god, he can do what he wants.  no one questions him.  When she gets sucked into the dream, that’s when he realizes that the feeling he has been feeling all along is love.  He Just didn’t know what to call it I think because he has never experienced it.  I think that’s why, in the show, when Ji Hengs dad dies, he points out to DH that he won’t understand his actions because he has never been in love.  Love makes no sense.  drives You to do crazy things.