Watching Tom Hagan in Godfather I and II, I see three different men.

Tom Hagan #1: consiglieri to Don Corleone - matter of factly stated his business, no opinions, just respect and the business at hand.

Tom Hagan #2: consiglieri to Santino - glib, quick and angry frustration, saying anything he wanted without much fear

Tom Hagan #3: consiglieri, or just "our lawyer in Vegas" to Michael - quiet, polite, a Yes Man, in total fear of Michael.

Tom Hagan's relationship with Vito Corleone was that of a respectful son.

Hagan's relationship with Sonny was that of an equal. He didn't fear Sonny because he knew that Sonny loved him like a brother, and felt he could say his honest opinions without fear of repercussions. I was always amused how Hagan would sometimes yell at Sonny in front of everybody - he didn't show Sonny much respect at all.

Now, Hagan's relationship with Michael, well. Hagan wouldn't dare shout at Michael, especially in front of other people. Hagan became very subdued, knowing that Michael could have him killed if the spirit moved him. Hagan knew that, deep down, Michael considered him as a brother, but also knew that Michael wouldn't take any shit from him.