For explanation of prior events, you could look at summary on the Wikia. This will be more focused on dialogue between Emilia and Roswaal.
This is the part just after Garfiel's defeat at the hands of Otto, Ram, Subaru, and Patrache.
Although I couldn't leave out this golden one.
When Garfiel defeated both Otto and Ram, he threatens Subaru that he ripped and tear both apart, which Subaru doesn't believe at all (Garfiel cannot kill anyone unless in full beast-form).
Subaru claims Garfiel couldn't have killed both Otto and Ram. Garfiel's reply was.
[Garfiel: What makes you think I didn't kill Otto? Although Ram could be exception]
[Subaru: Yes. Ram could be exception.]
[Emilia: Both of you have some kind of grudge against Otto-kun?]
Anyway let's start the main part.
When Subaru and Garfiel are both unconscious on the ground (Emilia/Patrache nearby), Roswaal appears. Emilia is on alert, because lesser spirits wavers and Emilia knew Roswaal was waiting behind when Subaru/Garfiel were making a showdown.
Roswaal comments about how weird world has become with Earth Dragons domesticated, saying he used other animals in the past. When Emilia says she didn't know that even she is older than him, Roswaal sneers at her.
[Roswaal: I must have something animals don't like. If it also applies to spirits, I understand why Beatrice doesn't like me]
Then they get to the main point.
[Emilia: Answer, Roswaal. What was your intention on Subaru?]
[Roswaal: You can't make proper decision even now. How fool are you to cling on other's good will. Even when everything you had before were the opposite.]
When Emilia was about to use magic to intimidate, Patrache stops her and Emilia realized this is not the time for violence. To which Roswaal is disappointed and says [I really don't have any good memory with Earth Dragon]
Emilia notices Roswaal was almost willing to be attacked by her, but nevertheless is worried about Roswaal. His face just looked like given up everything to her.
[Emilia: If you are worrying about something, tell me. I don't know what it is but I will be definitely helpful. I have been helped out by you many times...]
[Roswaal: Stop. It doesn't matter, Emilia-sama]
[Emilia: What do you mean by 'It doesn't matter'?]
[Roswaal: Just as I said. I don't want you to understand me. Those two, the Trial, or even Royal Selection - Who cares? This world is already over]
[Emilia: World is over? Who care about Royal Selection? What are you saying now, Roswaal!]
[Emilia: You and I started this whole mass. How could you give it up in middle?]
[Roswaal: You and I? Who said that?]
[Emilia: What?]
[Roswaal: My master and I started this. You belong to nowhere, and it is me and my master who have right to end this.]
[Emilia: Even if you and that someone started this, it is no longer only your or my problem. We already dragged so many people in this. You can't finish this just as you want!]
[Roswaal: What meaning is in this world anyway? We did nothing this time. Let's expect next Subaru-kun and us will do better than this.]
Emilia doesn't understand what Roswaal is saying and defends Subaru's effort, which Roswaal deemed useless when there is true history written, taking out his Book of Gospel.
He insults Emilia about her dependence on Subaru, then discusses about his concept of love, how him and Subaru is same on love - looking at only ideal parts of their lover.
[Roswaal: He doesn't care if Emilia-sama is actually weak, soft, and pathetic, right? He's not interested in anything about your true self. He's in love with ideal-you in his mind. Isn't it right?]
[Roswaal: I was same. Looking at her only ideal sides, always praising her and loved her like admiring a glass art - when it didn't have any meaning.]
[Emilia: That's it?]
[Emilia: If that's all you wanted to say, Subaru and you are completely different. He told me I'm bothersome.]
[Roswaal: What?]
[Emilia: He told me I'm nuisance, buried in the past, all talk, can't do anything without Puck.]
[Emilia: Subaru is definitely looking at me, and I am trying not to show him my weakness. So, Subaru and you, Roswaal, who never look at present instead pretends to see the future, are completely different.]
[Roswaal: Natsuki Subaru made correct choice in the Sanctuary? How could it be?! Then Book of Gospel is...]
Emilia continues what she thought about Subaru's love, and Roswaal is genuinely shocked first time in the story, shouting every words.
[Roswaal: F---!]
[Roswaal: Why? Why! How could he got that answer here and now?! How could Natsuki Subaru tell you which I couldn't tell my master?! Him here! This time! How?!]
[Emilia: I just know what I have feared. Gathering resolve to face it in time. What are you fearing about?]
[Roswaal: I already know! Deviating from the Book! Unable to meet her again because I couldn't follow it! What else I would fear!]
[Emilia: Your fear right now seems different.]
Em0ilia analyzed Roswaal wanted to put Subaru in his own shoes, follow his past, and snapped because it wasn't the case.
[Roswaal : I am being played around from time I made the bet? Garfiel defeated, Emilia regaining confidence, it was his plan all along? If Wise Man's deterrence is the case, was my plan all wrong from the start?]
After those ranting, Roswaal regains his confidence because he still had some cards left.
He leaves after saying Emilia could do whatever she want, doesn't care whether she completes or fails the Trial. And also warns her about Garfiel's stubbornness.
