Apparently, things got a little dangerous while they were filming P.S. I Love You — and Hilary actually ended up in the hospital!
"She was so cool, so great to work with, that I almost killed her!" Gerard said during an appearance on The Drew Barrymore Show.
Gerard explained that it all went down while they were filming the bedroom scene where he dances around the room in boxers and suspenders.
"I literally shot that scene for a day and a half and had to dance like an idiot in the suspenders...and I was like, 'I can't believe I'm doing this,'" Gerard laughed.
After successfully filming the scene quite a few times, Gerard recalled that everything went awry when the suspender clip got stuck.
"The clip, which was a crocodile clip, got stuck in the television as I'm crawling towards her," he shared. "It gets stuck, it releases, flies over my head, hits her in the head, slashes her head."
He continued, "I cut her open. You could even see the teeth [of the clip]. She had to get taken to the hospital."
Gerard says in a matter of "three seconds," the set was totally cleared out and he was left incredibly upset in his boxers.
"I'm just sitting there in my Irish boxer shorts and boots and a pair of socks and I just started crying," Gerard admitted.
He added, "I was like, 'I scarred Hilary Swank. I almost took her eye out, and I just made a fool of myself for two days!'"
Thankfully, Hilary was okay and it sounds like they were able to resume filming!
You can hear all that Gerard had to say below.