'Dirty Grandpa' Actor Feared Robert De Niro Was 'Going To Kill Him' on Set

Zac Efron and comedian Jason Mantzoukas were both guests on Jimmy Kimmel Live! on Thursday night, promoting The Iron Claw and the new Disney Plus series Percy Jackson, respectively. But coincidentally, the two had previously worked together when Mantzoukas made a cameo in Efron's 2016 comedy Bad Grandpa—also starring Robert De Niro—which gave the How Did This Get Made? host the perfect tee-up to tell an anecdote about working with the legendary actor.

And from the sound of it, the experience was terrifying, to say the least.

As Zouk, as he's known to fans, explained, he had been brought in for the afternoon to shoot a brief scene with De Niro. "I was like, 'Holy sh-t, this is happening, this is everything I wanted,'" Mantzoukas said of working with De Niro. "They were like, 'You know, we want you to come in and mix it up and improvise. You know, be funny, do your crazy weird guy stuff.'"

As he explained, when he showed up to the set, De Niro was initially not there for the setup and director's blocking. "Because we don't want to waste Bob's time. Let's be very clear you do not want to waste Bob's time," he told Kimmel. "In my mind, I'm like, I'm about to do a scene with Travis Bickle, I'm here to do scenes with De Niro."

But that enthusiasm quickly turned to pure terror. "Anyway, they come, they put him there, and then we just start going but I'm not doing lines at all," Mantzoukas continued. "The scene is like 15, 20, 30 seconds long, I'm going for two, three minutes, just looping around to nonsense, and he's just there like, stone faced, giving me nothing. And how much attention had been given to not wasting Bob's time is just playing in my head. I'm like, 'This guy is going to kill me.'"

"And we're doing take after take of this, and he's just like, stone faced," he quipped, mimicking De Niro's face at the time. "So we do this for awhile, and in my mind I'm like, 'I am for sure getting fired.' There's no way I'm not going to get fired, because this is nothing but a waste of time, this is indulgent, and—again, I had just arrived. They had been shooting for weeks and I just dropped in and I'm like, 'I'm going to do this now,' [mimics fart noise]."

"Anyway, so we do the whole thing and De Niro's giving me what I perceive of as 'I'm going to murder you' eyes, Goodfellas eyes, Raging Bull eyes, he's just absolutely going to destroy me," Mantzoukas recalled. "And as we finish, as we're exiting, he just goes, 'There was some funny stuff in there.'"

"I did it, but I mean, he gave me nothing," he added. But despite the terrifying experience, Mantzoukas said it's now a personal career high. "I swear to god I genuinely was like, 'That's it. I don't care if anybody ever appreciates what I do again.' I just got a 'There was some pretty funny stuff in there' from De Niro."