Kai Owen has revealed that Miracle Day will have the "most incredible end" in an interview with the Chicago Tribune.
It's always surprising. And I'm right with the viewers eager to find out how these things will get resolved. Whenever we would get an episode [script] I'd flip through it straightaway to see how it ends.

From Episode 5 on, my God, it goes into a rollercoaster ride and climaxes in the most incredible way possible. Only Russell T Davies could come up with it. And obviously, the fantastic Jane Espenson, who is working the majority of this season with Russell. If people are gripped now, they’re going to be even more gripped watching the rest of the episodes…

How does Russell’s mind work? I hope he wants to do another season, because obviously he has another story. But how does he come up with them? We all want to do another season. What will he do? What will he write? It’s so exciting.”
He also revealed how filming in the US was a "dream come true":
Warner Brothers, it was like a dream come true. We were filming on a [sound]stage that was constructed in 1935, that "Casablanca" was shot on. It was where "Two and a Half Men" had been shot--obviously it was shut down at the time--and where "E.R." stage that was home to Mekhi for eight years. We saw the "Friends" stage; it was like a dream come true. We were still making "Torchwood," but we were filming in Hollywood. It was bizarre. It's like a big dream now. It was amazing.
You can read the full inteview with Kai Owen here.