from what i can recall during the days of casting Welling did meet with Singer and he was "back-up", but it complicates things since Welling is in the middle of a very successful TV series and they dont want to end that (revenue$$$). thus they try to find another suitable candidate for backup, something the studios picked-- Caviezel, who doesnt have TV commitments and has just come off a very successful movie himself-- Passion of the Christ. the comparissons between Superman and Jesus Christ only supports Caviezel's bid for the role. but Singer was heard that he's looking into younger candidates who literally nobody has ever heard of. it took a long time and word starte to spread that if Singer doesnt make up his mind after a certain time period the studios will go ahead and sign Caviezel. then word leaked out that Singer has his pick. but the security around this actor was so tight so the name never came out until WB was ready to announce him.
then one day, a pictue came out ofa guy in a black t-shirt. and a name: Brandon Routh.
the rest is history.