It turns out that he found out his comeback to the HBO series not too long after his previous show, The New Normal, went off the air.
"I was very fortunate. Moments after The New Normal was cancelled, I got a very nice message from Lena Dunham saying 'please come back to the show,' " Rannells said. "So I felt very taken care of, she's a good, good friend so I'm very grateful to that."
"I'm happy to say that I get to join them," he added. "I get to join the cast of Girls next year full-time so I'm very happy about that."
Speaking about his character, Elijah, Rannells said, "Elijah's trying very hard to get back in with those girls because he didn't leave on the best of terms. So he's trying very hard."
The actor also talked about the harsh criticism Girls receives for its abrupt sexuality.
"From my perspective and sort of a lot of the criticism that I have heard are from people who aren't ultimately that familiar with the show, and sort of don't like it on a principle, which doesn't make a lot of sense to me," Rannells said.