I've been thinking some about the difference between the "Fredo" we see in the movie, and "Freddie" as he's mostly called in the novel. Fortunately both versions contain the "Don't ever takes sides against the Family again," line, so with either interpretation of the character you can still go on to GF2. I'm wondering, though, who could play the tough-but-dull Freddie of the novel. I actually think Danny Aiello would be very good, except that he was in his late thirties at the time the first movie was made.
"All of these men were good listeners; patient men."
I think Aiello would be too strong and assertive for Fredo/Freddie, and Pollard would be too goofy.
I'd reach into "Goodfellas" because Scorsese also was a master caster of Italian-Americans. My choices would be Johnny Williams (Johnny Roast Beef) or Tony Darrow (Sonny Buns), both of whom played occasionally clever but weak characters.
Ntra la porta tua lu sangu è sparsu, E nun me mporta si ce muoru accisu... E s'iddu muoru e vaju mparadisu Si nun ce truovo a ttia, mancu ce trasu.