IESB have posted some backstory to the attachment and unattachment of Superbad star, Jonah Hill:
You know how Jonah Hill was attached to TRANSFORMERS 2, and then was suddenly and without explanation un-attached? Apparently, Jonah was more or less offered the part in the room, but then had to go into Bay’s office to read the script — because it’s so super top secret and the plot is so extra-specially complex, you know.
Jonah sits down, opens the script. The character of the chubby sidekick is named “Jonah.” How perfect. Jonah reads 20 pages of the script, puts it down, and silently walks out of the room. Wants no part of it. Next thing you know, his reps call Bay, he’s off the project.
I’m thinking this may be because the chubby sidekick character was just too stereotypical with a whole host of terrible one liners and very little depth. Last thing you want to do is get typecast.