The Fabrizzio story is another example of changes FFC brought into the novel and another example of novel’s superiority to the script.
In the book everything is related clearly, even if indirectly.
Mike was secretly transported to the heart of province controlled by Don Tommasino, who was “Greatly indebted to his father for some past service”. He lived in a walled estate, and nobody knew who he was. Of course nobody ever told shepherds anything like his being a son of a big shot, it was a stupidity that no Mafioso would allow himself, all what is said: “of course it was impossible to keep them in the dark about his true nationality. Still they did not know exactly who he was except that he was in hiding and there could be no babbling about him.”
But Barzini is looking for Michael, and he has connections with the Sicilian Mafia too, it is said many times in the book. He must know that Don T. was greatly indebted to Vito, and expects that he must be the one to hide Michael. Through his friends in the Mafia of Palermo he organizes troubles for Don T. It is said that after about 7 months of Mike’s hiding DT begun to have problems with “young Turks from Palermo", and they didn’t wish to be reconciled by any concession he proposed, but with strange stubbornness continued the feud and tried to kill him several times. It was intended not only to find out if Mike was really there, they must have learned it already after 7 months of spying, but to disguise his future killing.
Remember what DT says to Mike when he comes to consciousness: “The Palermo people have made their peace with me now that you’re supposed to be dead, so it was you they were after all the time. They wanted to kill you while making people think it was me they were after. That’s something you should know.”
Even before Sonny was killed, and Vito told the Dons at the peace meeting that he will newer forgive his son’s death, Barzini knew him well enough to fear his revenge.
When Michael is at home, he has such dialogue with Vito:
Vito: I was surprised, though, that Barzini still made a last try at you. Maybe it was arranged before the peace talk and he couldn’t stop it. Are you sure they were not after Don Tommasino? Michael said, “That’s the way it was supposed to look. And it would be perfect, even you would never have suspected. Except that I came out alive.”
So, Michael’s murder was long and carefully prepared, and, by the way, Mike revealed his presence marrying Apolla. So the only thing left to do was to find actual assassin, who could get close to Mike, because there was no way to get through the army of bodyguards surrounding DT and the Villa.
In fact they didn’t have much choice here. It couldn’t be Dr Taza or Filomena, so they had to choose between honest Calo, “a simple fellow, almost moronic, silent as the dead”, and Fabrizzio, who was a famous guy in his village because of his tattoo, more witted and outgoing, and obviously more lightheaded. He couldn’t possibly refuse them, because it meant his death, he couldn’t stand alone against Palermo Mafia, and he had no reason to be as faithful to Mike as to accept death, instead of money and move to America.
His disappearing was important for Barzini, because if he would be found and tortured well enough by Corleones, he might tell that he was hired to kill Michael, not DT. So he was hidden well enough not to be found, because it would be difficult to find him if Mike would die. Nobody would know that he was not blown in the car with Calo and others, and that he was the one who brought the car from the garage. Strange, by the way, that Barzini didn’t simply kill him, and no problems whatever. But Michael survived, and he knew whom to look for, so it didn’t take long. In the dialogue cited above Mike continues with:
“Except that I came out alive. I saw Fabrizzio going through the gate, running away. And of course I’ve checked it all since I’ve been back.” “Have they found that shepherd?” The Don asked. “I found him”, Michael said. “I found him a year ago.”
Of course Fabrizzio was not unprepared for their proposal, and of course Mike was the first person to put the thought in his mind, when he said that there were people who would pay a lot for information about him. About his wish to move to America Puso never says a word, it is only said that from time to time he would joke with Mike and ask questions about America. Maybe that’s where FFC got the idea, but he exaggerated it and showed it primitively and bluntly. In the book Fabrizzio had his own additional reason to have Mike killed:
"Fabrizzio said “Is your wife coming with you?” Michael squinted down at him. It occurred to him that Fabrizzio had been following Apollonia too much with his eyes the last few weeks. Not that he would dare ever to make an advance toward the wife of a friend of the Don’s. In Sicily there was no surer road to death. Michael said coldly, “No, she’s going home to her family first, she’ll join us in a few days.” He watched Fabrizzio hurry into the stony hut that served as a garage for the Alfa Romeo."
So, if Michael dies, Apolla is not supposed to be desperate for life, and anything is possible… Besides, Fabrizzio had no way to know that Apollonia was pregnant.

I believe I should ask Don Geoff to measure our posts in meters instead of quantity, how do you think?