The actor had to read both Eminem’s lines and lines for his character, and he could not take either of them seriously.
Rogen told this story in his autobiography “Yearbook”. Seth’s film career did not take off back then, and he would audition for any role that would come through his agent, even when he did not fit into the project at all, as it happened with Eminem’s hop hop blockbuster.
Apparently, because of how specific the language was, a casting director did not feel comfortable feeding lines at auditions and asked all actors to bring somebody who could help reading lines for the opposite character.
Seth Rogen teamed up with his friend Jason Segel, who auditioned to the same role on the project, B-Rabbit’s friend Cheddar. Insider quotes this section extensively, giving the taste of the situation:
Auditioning is embarrassing in the best of times. Add the fact that one of my best friends is watching me do it and that we’re both reading for rappers from Detroit, which we could not have been less right for.
Young actors had to read a short exchange that Seth Rogen remembers as “Yo, yo, mothafucka! It’s Chedda! What up, bitch! — Yo, yo, Rabbit! You gotta record your shit at Paisley Park, yo! — Where, yo? — Paisley Park, mothafucka!”
Seth Rogen could not handle it:
I started laughing hysterically. And so did Jason. We literally couldn’t make it through the auditions. As soon as one of us started the scene, the other would lose it… It was so silly, we couldn’t finish. We just excused ourselves and saw ourselves out, tears streaming down our faces.
It seems like yo’s and whadup’s felt too ridiculous for two aspiring actors fresh off New-York intellectual comedy scene. The role eventually went to Evan Jones, who did a pretty good job portraying a young man for whom poverty and the lifeline that rap threw to the new generation was not a joke.