Noah Wyle knew it was a huge departure from his tenure as Dr. John Carter on “ER” when he signed on as professor-turned-warrior Tom Mason in TNT’s sci-fi series “Falling Skies.”

While the role doesn’t involve complex medical jargon, it does require plenty of action.

“I hadn’t played many physically dynamic characters before, and had never started a show as the center of an ensemble,” he says. “I sort of inherited ‘ER’ over the years as other characters departed.”

But what sold Wyle was not the physicality of the role, but whatmakes his character internally tick.

“I responded to the throughline of fatherhood in his character,” he says.

Wyle knows spaceships and aliens are fun, but it’s flawed characters that really drive the show.

“The profound sense of loss all of these characters have to grapple with was also interesting to me,” he says.

Also, the idea of deconstructing and rebuilding a character that really fascinated Wyle.

“Gradually he realizes his historical preferences actually could be a tactical strategy, and he begins to find his voice,” he says.

Mason battling aliens is a world removed from Carter saving lives in the ER, and Wyle is having fun with it.

“I traded a stethoscope for a very heavy machine gun,” Wyle says.