...but he failed to show up for that one day's shoot. So Coppola rewrote the script.

I guess we don't have the original script to look at, but I'm assuming all of Sonny's words to Michael were gonna be Vito's words to Michael?

If so, then Sonny's screen time would have only been him introducing Carlo to Connie (in 1941 - then married in 1945...damn, that's a pretty long courtship for the 40s), hardly worth the big salary Paramount promised him.