"He can't kill me because he needs me for his plan. I cannot be imprisoned. What good do you think would have come if I started a country at his borders and fought him? His country is built to be at war indefinitely and I would have fueled it and put other innocent lives at risk."

"The time you have to prepare, he has also. If he needs you and knows you would oppose him, then he has in mind a way to hold you. Do you know what it is?"

"He needs to prepare his plan, which has far more moving parts. His plan for me ... when I was last in Amestris I felt that the Dragon's Pulse connected to the earth was disrupted. When I studied with him, hundreds of years ago, alchemy was performed using the ground as a conduit and distance arrays were thought to be impossible. I think he believes himself capable of rendering alchemy unusable in his country by disrupting the connection to the ground. But even if he does, the ability to sense the Dragon's Pulse will be enough to counteract it."

"And his servants? Wrath, Pride, Lust, Greed, Envy? What if he has trained alchemists of his own and sets them against you? Or his army?"

"He needs me alive for his plan. I am actually in less danger than most anyone else in Amestris or otherwise."

"Remaining alive is the lowest goal. How do you intend to win?"

"Homunculus's plan is to open the world's Gate and incorporate all the stuff beyond it into his terrestrial body, but he won't be able to acquire it without a large initial energy cost, and he won't be able to maintain it in such a small space without consuming souls. If I reach him as he makes his initial energy transmutation, I and the souls in this body can use the array he is composing to put the country's citizens back. Then his body will be unstable and the only concern will be limiting the fallout as its energy is released and the body dematerializes.

"I admit I don't know how long his body will last afterwards. He may have enough souls to live for years unless he is physically killed. But after the transmutation he won't have a stone, and he won't be able to use me."

"So when he realizes what you've done and that you are of no further use, he will kill you. And you hope that the damage he does in his last flailing is more survivable than what would happen should he succeed."

"If he succeeds he kills fifty million people initially and prevents new sapients from being born for eons, while preventing anyone in the country from using alchemy but him. Then he leverages that power into consuming all life on this planet. Given how freely he spends souls he'll eventually run out. Once I was thankful that at least then he would die and the soulstuff would be released, but now that I know he could travel to a different universe altogether ... any flailing he does in his last minutes -- hours -- years -- will be nothing next to that."

"An eventual end is not enough. The error you are making is that you cannot settle for disabling your enemy. You must want to destroy him utterly. If he turns you aside as you are now, you will achieve nothing. But if you aim for eradication and he deflects part of your attack, you may still have a chance at succeeding. Focus your entire desire, all of your effort, upon this, or you will fail."

"The eventual end is a worst-case scenario. Even being one of his sacrifices is not ideal. If I can do something to disrupt his plan before then ... but that's never been feasible. Until now."

"Never accept the worst-case, even when it comes to pass. Fight against it, to your dying breath and beyond. That is the attitude you must hold, and it does not seem to me that you do."

"The worst case will not come to pass if I spend enough time on this plan. My efforts don't have to look like fighting to be useful."

"It doesn't matter what your efforts look like. It matters what the feeling behind them is. 'Through passion, I gain strength'."

"I suppose if it works for you. You'll forgive me if I'm less inclined to adopt it myself. I've made enough mistakes in passions."

"If you allow your passion to rule you, you will make mistakes. But if you harness it, it will provide you with the fire necessary to see your task through."

"You shouldn't so quickly discount motivating emotions that are different from yours."

"I care not what you feel, so long as you do."

"On that we can agree."

"But do you?"

"I don't find righteous passion as useful as you seem to."

"Hm. Enough, then."

They retire for the night.

And they visit the other healer the next day?

Yep.

In the morning Hohenheim shows Occlus where the Shahidi household is. He also explains how he used his chi sense to find them specifically. Not something he expects her to be able to reproduce immediately, but definitely something to keep in mind.