If Horatio were able to talk back to Hamlet what would he say and how did Hamlet's madness affect him?
Horatio's last words to Hamlet speak volumes about what he thought of Hamlet's supposed madness. "Never believe it./I am more an antique Roman than a Dane./Here's yet some liquor left" (V.ii.4000-4002). After Horatio witnesses the true madness unfold--Laertes mortally wounding Hamlet, Hamlet mortally wounding Laertes, Gertrude drinking the poisoned wine, Laertes uncovering his and Claudius's plot to kill Hamlet, and Hamlet feeding Claudius the poisoned wine and then stabbing him with the poisoned sword--he wishes to join those who perished and are about to perish by drinking what's left of the poisoned wine. But Hamlet doesn't let him. He begs Horatio to live and set things right, to tell the people of Denmark Hamlet's true story: how King Hamlet was murdered by Claudius, and how everything that followed was enough to drive a man.
But did Horatio think Hamlet was mad? After all, didn't Horatio see King Hamlet's ghost as well? If Horatio could answer Hamlet's request to tell his story, I think he would tell his friend that no man should have to endure the tragedy Hamlet did--the death of his father, the marriage of his mother to his father's brother (and later, known murderer), Ophelia's rejection of him, Ophelia's death as a result of Hamlet's hasty murder of Polonius--without losing his own mental stability.
I think Horatio understood that even though Hamlet claimed he was going to play at being mad, too much tragedy befell him. It was all too much for Hamlet to handle, and his acting mad spilled over into real life. Horatio would have said to Hamlet what he said to the witnesses of the Elsinore massacre at the end of the play: "So shall you hear/of carnal, bloody, and unnatural acts;/Of accidental judgments, casual slaughters;/Of deaths put on by cunning and forced cause" (V.ii.4047-4050). Horatio would have admitted to his friend that Hamlet was responsible for all acts described here, but so was everyone else who perished.
Horatio is honest and noble, and he would have stood by his friend, understood that his madness was more than just play-acting, and helped him see how the chain of events that happened from the play's opening led to the tragic final scene.