It is kind of an odd scene, Pete. I think Buscetta waiting outside was Michael's way of putting Mrs. Pentangeli at ease--he's a pretty scary looking guy. And, if he'd gone in with Michael, probably some of Frankie's bodyguards would have had to come in, too.

Buscetta enters the story out of nowhere. But, he would have had to have had some connection with the Corleones to turn up on such short notice (the train scene). Frankie spotting him and saying, "We got company" affirms that he was a known commodity.

Oli, good catch about the wall. I also didn't think Frankie's office looked much like Vito's in the Connie wedding scene.