This article has some more info.
It later emerged that in May 2017, just before Trump fired Comey, Trump and an aide drafted an “angry, meandering” termination letter to Comey at the president’s golf club in Bedminster, New Jersey. The Bedminster letter was never sent. But in a meeting in the Oval Office soon after it was drafted, Rosenstein “was given a copy of the original letter and agreed to write a separate memo for Mr. Trump about why Mr. Comey should be fired,” according to the New York Times.
In short, before the Schmidt story, we knew that Rosenstein was intimately involved in the president’s decision to fire Comey. Rosenstein’s memo was used as a pretext to fire Comey; Rosenstein knew that the president wanted to fire Comey; and he read the Bedminster draft before he wrote his own memorandum.
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Schmidt’s story yesterday added two details that make it even harder to see why Rosenstein does not have a conflict. First, Schmidt reports that, contrary to White House claims, Trump’s Bedminster letter mentioned the Russia investigation in its first sentence and described it as “fabricated and politically motivated.” Since Rosenstein saw that letter before he wrote his memorandum—and perhaps even discussed the contents with the president or attorney general prior to drafting his own second memo—he knew that the president had the Russia investigation on his mind in wanting to fire Comey. He wrote his memorandum about Comey in light of this knowledge.
Second, after reporting that Attorney General Sessions “wanted one negative article a day in the news media about Mr. Comey” and that a Sessions staffer approached a Hill staffer about whether he or she was in possession of any “derogatory information” about Comey, a claim denied by the attorney general’s spokesperson, Schmidt added:
Earlier that day, Rod J. Rosenstein, the deputy attorney general, had pulled one of Mr. McGahn’s deputies aside after a meeting at the Justice Department. Mr. Rosenstein told the aide that top White House and Justice Department lawyers needed to discuss Mr. Comey’s future. It is unclear whether this conversation was related to the effort to dig up dirt on Mr. Comey.