Important moment - lighting Enzo's cigarette
I'm rewatching and I love the part where Michael is at the hospital and he realizes hitmen are coming to kill his father. He sees Enzo the baker come in and tells him to meet him out front. They stand there and try to look like guards. When the hitmen show up Michael reaches into his pocket like he has a gun and they take off.
Then Enzo tries to light a cigarette but he's shaking with fear. Michael calmly takes the lighter and lights the smoke. He stares at his hands and realizes he isn't shaken. His nerves were steel, he was calm, collected, and resolved in his actions. He's realizing he's built for this life. In the next scene, he's sitting in the chair with confidence and is ready to take the next step - planning the hit on McCluskey and Sollozzo.
It's these little moments in the script that really make this movie a standout.