Tom Hagen is never called anything but "Tom" in Parts 1 and 2. Never Thomas or Tommy. It's about 150 times, including stage directions, per Geoff's transcripts. In the novel, he's called "Thomas" the very first time he's named by the narrator, and then called Tom every time thereafter, including in all dialogue.

Tom is referred to as "Tom Hagen" at times when it seems unnecessary, e.g., "Tom Hagen is no longer consigliere," when he's present and everyone in the room knows him. In the movie, Kay got, "...something for your mother, and for Sonny, and a tie for um um Freddy, and Tom Hagen got the Reynolds pen..." That can only reflect how Michael
habitually refers to him. The narrator in the novel runs amok with this, often calling him "Tom Hagen" multiple times in quick succession. In the section where Tom is arranging for Lucy to go to Las Vegas, Puzo calls him Tom Hagen in three consecutive sentences.

All true, Pete. And, Neri is almost always referred to as "Neri," seldom as "Al Neri," and never (if I recollect) as "Al"; whereas Rocco is almost always referred to as "Rocco Lampone."
On the Trilogy boards, nearly all posters here call them "Neri" and "Rocco" and "Tom."

The only time I remember Neri being called Al is during the scene at Tahoe when Connie is telling mike of her plans to marry Merle. Merle asks for a drink and Connie yells "Al will you get him a drink?"

True enough Pete

When Michael returns from Cuba, at the motel he asks Al to get him a towel and then thanks Al.

However, although neither the novel or GFI anticipated a GFII, Michael's almost constant reference to Tom as Tom Hagen reflects what I've always contended is Michael's blaming Tom for the Corleone's misfortunes. But it begins even sooner when Michael introduces Tom to Kay as Tom Hagen and then says (needlessly) that Tom is not a Sicilian. At that point there is no blame, but it illustrates Michael's glandular discomfort with Tom.

Oli, I agree that Michael blamed some of the family's misfortunes on Tom. He said it all when he said, in the fishtank scene, "You're not a wartime consigliere, Tom--things could get rough." In the novel, after learning about Sonny's assassination, Tom himself realizes that he's "not a wartime consigliere--old Genco would have smelled a rat."

But, at Connie's wedding, Michael introduces him as "my brother, Tom Hagen" [emphasis added]. That's the innocent, not-part-of-the-Corleone-crime-empire Michael talking. In II, after the Tahoe shooting, Michael (in the hyper-manipulative mode) says, "You're my brother, Tom." Poor Tom practically breaks down in tears and says, "I always wanted to be considered as a brother by you"--meaning that he always doubted that Michael considered him as a brother.

Hagen's occasionally directly addressed as "consigliere." In nearly all those instances, I think there's at least some measure of irony, ranging from Vito's "Consiglieri of mine" to Sollozzo's "Aw, make time consiglieri."

TB, I think that the wedding introduction of Tom to Kay was for the audience's benefit. Not everyone in the audience had read the novel, so the film script let's those audience members know Tom's relationship to Michael. The same is true when Michael introduces Fredo to Kay. You might note that when Michael introduces Tom to Kay he afterward adds that Tom is probably going to be named Consigliere. He does so without enthusiasm. But that part of the scene is telling because Michael adds that Tom is not a Sicilian. While that is true, it's unnecessary for Kay to know.

As we've opined in other threads, when Michael speaks with Tom after the attempted murder of him, Michael is simply manipulating Tom with his brother comment. Realistically, Michael doesn't (and can't) trust anyone else. However, he knows that Tom is loyal to Vito (and thus the Corleones) to the end. So, Michael's referring to Tom as brother in that conversation is simply Michael's manipulation of Tom's loyalty when Michael has noone else to turn to.