In the novel, during the meeting between Sollozzo and the Don, the only other people present are Sonny and Hagen.
To met this makes more sense than the film version. I know it has been argued that Vito wanted everybody else to know that his no to the deal was strict. On the other hand, it was a delicate issue. Nobody could foresee if Sollozzo would come up with something else, an alternative plan.
Vito avoided witnesses. Even Clemenza and Tessio received their orders via Hagen.
And as we saw, even Sonny was one person too many. Vito could have known because of Sonny's behavior during the wedding and elsewhere, but perhaps he saw no alternative.
Opinions?

Hagen had to be at the Sol meeting because he set it up. Logically, Vito wouldn't have included Sonny if, as the novel strongly implies, Vito's mind was already made up to say "no." But, since Vito had included Sonny in the pre-meeting discussion, he had to be at the meeting. And, more important, Puzo needed to present us with Sonny's indescretion to set up the shooting, Sol's post-shooting proposal to Hagen, and Michael's descent into murder.

I chalk this up to FFC's dramatic license. Just like he had Sonny murdered by an array of machine gunners instead of just a couple guys with pistols (as in the novel), he arrrayed most of the main characters at the meeting for dramatic effect.

Anyone complaining? It was a great scene and that's all that matters.

Despite the history between Clemenza, Tessio, and Vito, Vito could never be sure of their loyalty (of course, Tessio proved him right). Thus, he would not want them in the meeting to preclude their enticement. The large Corleone profit (50%) that Sollozzo stated at the meeting certainly could entice one or both of them.

Sollazo was a great character in the movie. Luca was a great character in the book.

Movies have time constraints so they have to cut things out like Luca character.

. I think they had the whole family in the meeting with Sollazo because the don wanted to show a united front when he said no. It worked out badly for the Don because he did not talk it over with the kids before he met with the guy.

But then it was only a movie and not real.

If the talk was real he would have said yes because it was too much money to say no. It would not have had a big group of people their either.

Richie, we're talking about the novel here.