Also at some point the studio attempted to cast Billy-Bob Thorton as GG (according to Thorton) but he thankfully turned it down, saying it also wasn't his thing. Funny, Dafoe looks like was fourth or fifth choice, but he was the best guy of the ones listed for the job (I think Neeson was a good choice for Ock but not really Gobby and much better suited as Ra's Al Ghul). Also Sam Neil at some point was considered for Doc Ock in SM2, but couldn't do it. Jake G was obviously almost a Spidey replacement in the second film as well.
As for Peter in the first movie the studio wanted Freddie Prinze Jr. of all people but Raimi pushed it tooth and nail and pushed Maguire and actively sought out Maguire. He first had to convince Maguire to do it, and then he had to convince the studios to let Maguire do it, and Sam's back up was Wes Bently (from American Beauty and Four Feathers...and on a low note Ghost Rider's villain as well). Fortunately Tobey got the role. Supposedly it was offered to Heath Ledger though and MJ WAS offered to Kate Hudson.
As for Ock/GG. The plotline was basically Norman is a through and through ******* with little conflict of good and evil, even though he did struggle on how evil (more Mackie than Stan Lee here) who fired Octavious, working for Oscorp, for no reason than to save a few pennies. Ocatvious who is working on the performance enhancer gas with his arms gets pissed and goes crazy trying to kill Norman with the arms, this causes an explosion and Octavious is badly burned sticking the arms to his back and losing his research. Norman enhales the gas and his eyes turn green.
Norman then decides his son (who is written as a selfish little snot in this draft, completely unlikeable) deserves to die, because he cannot be trusted to run his grandfather's company. GG then tries to kill Harry Osborn at the festival (he is in no risk of losing his company) and is thrwarted by you know who. Octavious thought Norman is dead and recognizes the flight suit and glider and knows Osborn lives. Octavious is the one who attacks Jameson at the Bugle to find out who Peter Parker is (who does have a photo credit) to take him to Spidey...to take him to GG (as the Bugle says they are in cahoots), Peter says he doesn't know who GG is. Ock throws Peter across the room much like SM2 and leaves.
Spidey then at some point is given a stabbed needle injection like in SSM #200 by GG to join forces with him. Spidey later declines (I forget the details) and they are enemies. Norman tries to put GG in check and loves his son but then finds out at the Thanksgiving indecision meeting (Aunt May is not there, like in the whole movie, and the three kids and Uncle Norman are going to go out ot dinner) and thinks they all deserve to die when he finds out MJ is in love with Peter.
GG invites all three to Oscorp under false pretenses and tries to blow them up, Harry leaves Peter to die and nearly gets him and MJ killed by stopping an elevator with a glass wall GG can see so as to ***** her out for being in love with Peter (when she isn't, she is strictly in love with Spidey in this movie and Peter is the equivalent of Clark Kent to Lois Lane). Peter, who burned his costume after Aunt May got hurt, in middle of the climax goes and buys one from a costume shop across the street, saves Harry and MJ only to have his ass saved while holding the rope to the elevator by...Doc Ock who gets in a tussle with GG and takes off his mask and reveals it is Norman.
Spidey then at Harry's pleading fights Ock to save GG, who then tries to kill MJ, who Peter tags along for the ride to the bridge. Peter saves MJ, Koepp's screenplay is completely condescending about the climax and then saves GG as well. Ock is pissed GG lived, and Harry's darkside comes out when he sees MJ and Spidey kiss and is even more pissed (by how many exclamation points come after the yelling) than Ock. We then see Norman go to jail for life and Harry be a *****e bag again and then promise to get revenge on Spidey...for stealing his girlfriend. Ock is still out there doing God knows what at the end of the movie and Spidey ain't looking for him.
There is a brief summary of how Koepp put these two villains together. And people thought SM3 didn't work...