And we didn’t really establish a real character flaw in the first movie because the character who experiences growth in the first movie is George McFly.
Let’s use that as Marty’s character flaw, that he’s a hothead.
And so we can verbalize that by having him say “Nobody calls me chicken!”
Well, when it came time for us to do the sequels, we realized that we needed to give Marty a character flaw that he was going to have to correct.