"Dean accepting the angel's offer of going into his brother's body, that's essentially set up what will be a central conflict that will be driving the season," executive producer Jeremy Carver tells THR.

Will Sam (Jared Padalecki) find out about the angel that's healing him?

Supernatural returned with Dean (Jensen Ackles) making a deal with fallen angel Ezekiel (Tahmoh Penikett): Heal a dying Sam back to life and at the same time heal himself. What becomes of Dean's decision and what happens when Sam discovers the truth will serve as the major thread through season nine, significantly affecting the Winchesters' bond.

"Dean accepting the angel's offer of going into his brother's body, that essentially set up what will be a central conflict that will be driving the season," showrunner Jeremy Carver tells The Hollywood Reporter. "How Sam and Dean react to it is something we want the fans to see unfold. It's absolutely appropriate to ask that question and for fans to ask that question, and I think there are going to be a few surprises along the way in terms of how we answer them."

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But Carver assures that there won't be drastic personality shifts when Ezekiel or Sam come to play, at least not yet.

"We got a taste in the premiere that when Ezekiel is there, it seems to be Ezekiel, and for all intents and purposes, Sam is Sam," he says. "Sam is very much unaware of the angel that's inside of him. What's going to be interesting is watching how both angel and human begin to unwillingly interact with each other."

The person who will be most affected by the "new" Sam won't be Sam himself. It will be his brother, Dean.

"We're going to see this mostly play out through Dean, as he has to balance these two separate entities in his brother. That's where the weight of this whole thing lands," Carver says. "We're going to see very early on what it's like to be Dean, who is forced to knowingly balance Sam and Ezekiel."

Carver was coy about when Sam would discover that there were two beings living in his body ("That is something that is not yet to be revealed," he hints) and the secrecy surrounding Penikett's potential return was just as high ("Fair questions," he says, without offering any hints).

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The Winchesters will be dealing with far more than just the usual brotherly woes.

"Both brothers are going to be dealing with the landscape that we're setting up in the first three episodes," Carver says. "Trying to figure out what's going on with the angels, finding a way to put them back back in heaven, trying to counteract what's happening in hell and the various other personal arcs the brothers are going on."

With former angel Castiel (Misha Collins) now in human form, it will be a constant challenge for him to come to grips with his new state. The fact that the fallen angels are out for vengeance certainly doesn't help matters. "It will be difficult," Carver says with a laugh, teasing that Castiel will reunite with the Winchesters sooner rather than later, and may look for help in "unexpected places" versus the angels.

Meanwhile Crowley (Mark Sheppard) -- he's a "prisoner of the brothers" at the moment -- will factor into the first part of the season. Still locked in the trunk of the Impala in the premiere, "Crowley being Crowley," Carver says, will get out of it somehow.

Supernatural airs Tuesdays at 9 p.m. on The CW.