In "American Assassin," Michael Keaton shows no mercy while training Dylan O'Brien on how to be a counter-terrorism operative.

But in real life, Keaton, a veteran actor, and O'Brien, a former YouTube star, joke around easily.

Kim Holcomb talked to them about making the spy thriller, what audiences should expect, and O’Brien getting his start doing internet comedy skits.

HOLCOMB (to Keaton): "When your characters meet on screen for the first time, I believe your line is, 'You speak to me, you call me sir.' What was it actually like when you met him for the first time?”

KEATON: “That's the way it works - that's what I told him.”

O’BRIEN: “Just asserting his dominance out of the gate.”

KEATON: “He stuck with it for, like, nine minutes. Then he'd call me, 'Hey you!'"

HOLCOMB: "When you were lip syching to the Spice Girls [on YouTube,] which I enjoyed very much I must say…”

O’BRIEN (turning to Keaton): “Ear muffs, ear muffs."

HOLCOMB: "Could you ever have imagined you'd be in this movie, or in any movie like this, playing a role like this?”

O’BRIEN: “No. No, no, it's a really crazy thing. I did that as a kid for fun, I loved making little movies it was always something I loved -- funny enough, always something I loved so much, that I never thought I could actually do it."

HOLCOMB: “I’m tense from watching the movie.”

KEATON: "We have chiropractors in the lobby for every performance.”

O’BRIEN: “For the first weekend.”

KEATON (laughing): “For the first weekend only, can't afford past that… Advil. Everybody gets some free Advil.”

"American Assassin" is rated R and opens in theaters September 15.