I think Luca's portrayal in the movie was consistent with his character in the book. He was a man of action and a power, "like dynamite or lightning," but he also chose to serve Don Corleone, even though there seemed to be no compulsion for him to do so. Hence the contradiction between his character and the flowery words that weren't natural to him, and that he had to deliver in a stammering, awkward way. But he was smart enough "to do a job of murder all by himself." And the Don trusts him with a major threat to his life. "Go to the Tattaglias, make them think that you're not too happy with our Family, find out what he's got under his fingernails." That's not a mission for a guy whom the Don thinks is dumb, or is just plain muscle.