It was a slight surprise to find the Doctor actually doing what he'd been told. Then again, when confronted with the very real possibility of death if he didn't do what he was told, the Doctor would go for extending his life. Koschei went and sat next to the Doctor, hoping he could somehow help. Poisons weren't his forte, but he would do whatever he could to help keep his friend alive.
"Koschei is my name, so you can call me that." The name of the Master tended to confuse him, as he really didn't know why this other version of him liked the connotations of such a name. Koschei certainly didn't like it, he was more of a helper and observer than anything else. "Do you have your sonic screwdriver on you? We could start with that, see if it can tell us what exactly we're dealing with. Though I'm not so sure Earth has everything we need to combat a Time Lord poison." He was aware that some things on Earth could help, and he hoped the posion would be one of the somewhat treatable ones. The alternative was one he didn't want to think about.