Robert Duvall is good here in his role as a reasonable man. He shows Tom Hagan as not an unemotional man, but rather a man who keeps his head despite being threatened or otherwise. Hagan is a character who mostly does take care of business carefully, and Duvall properly shows Hagan as a level headed man.
Tom Hagan is the least complicated of the Corleone sons since he does always stay fairly mild mannered, only really raises his voice when he is trying to calm down the hot headed Sonny (James Caan). Although the nature of the role prevents Duvall from giving a great performance, his performance is more than satisfactory for the part, and still completely embodies exactly what Tom Hagan should be in the film. He keeps his character interesting despite being the most reasonable, Duvall has the right quiet persuasion and presence in the film.
Also in his few chances to show off a little more of Tom Hagan, Duvall does his best. Two scene in particular one where he must tell of some very bad news to Vito, is well handled in that Duvall shows Hagan more emotional side with the right amount sadness about the loss in the family. The other scene is when Hagan actually becomes a little more emotional questioning the loss of his rank in the family. A short moment but again well handled showing a little more of Hagan. Overall not a great performance but certianly a good one which simply adds just even more to the film it is in.