"Oswald has always been vilified in a huge way but no one has gone in to intimate moments in his life. I was definitely eager to get in to it," Rothaar tells PEOPLE of being cast as JFK's killer in the new National Geographic Channel movie Killing Kennedy.
"It was fascinating to dig deep and find out things about him that I didn't know."
In preparation for the film, which follows the lives of Oswald and JFK in the years leading up to Kennedy's assassination, Rothhaar entered an intensive study session where he, Michelle Trachtenberg, Rob Lowe and Ginnifer Goodwin delved deep in to their roles as the Oswalds and the First Couple.
"Everybody was wonderful. We were all sharing facts with each other," says Rothhaar. "Working with Michelle was wonderful and even though I never had scenes with Rob or Ginnifer I would come on set when they were shooting and I would just watch them do their thing. They embodied them so well. It was fascinating."
As for his getting in to his character, Rothhaar, 26, worked hard to find ways to identify with the man he calls "one of the most hated figures in American history."
"I came to the realization that he did not grow up with a lot of love around him," says Rothhaar, who previously starred in Battle Los Angeles and ABC's Last Resort. "His father died two months before he was born of a massive heart attack and his mother moved him around a lot. That screws up your foundation."
And Rothhaar was surprised to find the lighter side of Oswald as well.
"People say he was quiet and kept to himself but he was also very sharp and passionately articulate," he says. "And he was wonderful with his kids and the kids around the neighborhood. The people in his neighborhood loved him for that. That made him not a full-on monster."
Rothhaar is well aware of the controversy that surround Oswald and the Kennedy assassination, and he is prepared to face the conspiracy theorists who will be seeing him portray the killer when the movie premieres Sunday.
"I've gotten some really nice compliments on the film but I am prepared for some people to come at me with craziness," he says.
"One guy Tweeted me, 'You know this is a lie. He was set up. You're probably going to play him as a psychopath.' But I've also had some big conspiracy theorists come up to me and say, 'You know what, I've never really seen Oswald like that.' "
Chat Live with Rothhaar's Costar Ginnifer GoodwinJoin PEOPLE as we host a Twitter Q&A with Goodwin on Sunday, Nov. 10, at 7:30 p.m. ET, leading up for the premiere of the film based on Bill O'Reilly and Martin Dugard's book, Killing Kennedy: The End of Camelot. Follow us at @peoplemag and send us your questions for Goodwin, using the hashtag #AskGinny.
(You can also send questions in advance, so don't be shy!)
Ginnifer is @ginnygoodwin on Twitter and remember to include #AskGinny. You can also check out @natgeochannel for all the details about Killing Kennedy.