I agree with you guys that all those scenes are funny in some way. But do they really provide comic relief??
Honestly, did you laugh out loud when you watched the Fredo scenes for the first time. Or wasn't it that you smiled when you saw the scenes the second, third or 149th time?
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The landlord scene: Yes, it's a little funny to see the guy change. But we get the Chaplin effect mainly from the clarinet music in the background. I believe the scene is important because it shows Vito's commitment for poor people in his neighbourhood who seek for his help. Compare it to Michael who has to ask Tom about the birthday present for his son Anthony.
I think Signor Roberto was a caricature, especially if you compare his reactions to Bonasera or Nazzarine. I just don't believe the actor.