Bassett plays the director of the Secret Service during a major catastrophe.
"On the set every day, you know, all along the way, we had Secret Service personnel, those who were in it their entire career, and they were amazing," said Bassett.
In the film, terrorists capture the White House and kidnap the president. Butler plays a disgraced former presidential guard who may be their only shot at getting him back.
Bassett says the Secret Service agents she worked with were both genuine and warm.
"You feel so secure and yet you never would know that they had this completely other side of their life that…I couldn't imagine and didn't want to hear too many tales about it, honestly," said Bassett.
She was especially thrilled to have the opportunity to work with Freeman.
"I was intimidated before he even showed up, but there was absolutely no need to be. He was nothing but professional and kind and warm and fun and, you know, a treat," said Bassett. "At the end, I just couldn't help myself. I just, you know, grabbed him and said, 'Oh, I just had a ball.'"
"Olympus Has Fallen" is rated R. It opens March 22.
