Part of it might be the fact that I'm reading The Family Corleone and Luca isn't portrayed as stupid, but I sometimes have difficulty reconciling the fact that Luca is supposed to be this brutish, formidable figure with the actor who is playing him. Specifically, the way he stutters through his "thank you" speech to Don Corleone.

??

The Luca in the Family Corleone is based on the Luca in the original novel rather than the film, which makes sense.

I agree.

That's the risk you run. Are you writing a literary sequel? Are you writing a sequel to the films? Are you developing backstories for characters that were better left undeveloped?

Risky, risky business.

Could be: Luca Brasi was drunk off his head at the Corleone wedding, as he was afraid of talking to the Don (hence his rehearsing what he was going to say, out of nerves). In his meeting with the Turk, he doesn't seem like a dumb brute. Think about it this way, as well: We might be seeing Luca Brasi after his prime in GF I.

FYI: Even in the novel it mentions that Luca stuttered over his congratulations to Don Corleone. IMO it was done to illustrate that Luca was a scary looking sonofabitch but even HE was scared to death of Don Corleone.