"Yes," said Willie, "I'll take some orange pop." (1.129)
Does this sound like our Willie? Not too much. The post-alcohol Willie would never talk like that. This line speaks to his complete transformation.
"Gimme a slug." It was the Boss's voice. "You know where it lives." (1.227-228)
This is about ten years after the first quote we gave you. It's important because it shows how Jack and Willie are bound together at the hip (where Jack's flask lives) by alcohol.
"It's the first time," [Willie] said. "I never got drunk before. I never even tasted it but once before." (2.371)
Willie isn't quite hooked on booze yet. But the day is young. This whole scene is given too us after we know that the Willie of many years later has already made drinking a part of his daily life.