The actor is in awe of his 'Neighbors' co-star's hot body and admits he was often distracted during filming by admiring the 26-year-old hunk's chiselled physique.
Speaking on Conan O'Brien's chat show earlier this week, Seth enthused: "My body is like one big bump and [Zac's] is like 87 different little bumps that come together.
"It's like his whole body is a giant arrow that points to his [man parts]. It's like someone to draw your eyes down from his gorgeous face and suddenly it's like a Plinko game with your eyes."
And Seth, 31, was so impressed with Zac's toned abs that he asked the director to write in extra scenes where the Hollywood hottie could appear shirtless.
He added: "We had to keep writing into the scenes that I would touch his body, because I would look at it and just want to reach out and touch it, as though it was a mirage of some sort."
The 'Knocked Up' star is also jealous of how female fans react to Zac's presence and joked that he has never even managed to get his wife Lauren Miller to emit "one syllable" of the same high-pitched screeches.
He added: "They make a noise I literally never heard before in my life.
"You could give me three hours with a woman and I couldn't make her make that noise that they make. I've been with my wife 10 years. I haven't even gotten one syllable of that sound."