Hal, the Discovery's computer, is easily the most entertaining character in the book. Supposedly he's an unfeeling hunk of metal, but in reality, he's the only one in the novel who really seems to feel much of anything, or to act as if he feels much of anything. Bowman, Poole, Floyd, even Moon-Watcher all go about their business efficiently, setting goals and pursuing them. They may make tactical errors (like Moon-Watcher dragging the kill to the cave without realizing it would bring the leopard) but jealousy, pride, or other petty emotions don't distract them. They don't, in short, seem to have a subconscious. Everything for each of them is on the surface.
Not Hal, though. On the surface, Hal is an efficient, cheerful, friendly data-cruncher, always at the ready, always with the team, just like the rest of the other numbingly boring bureaucrats who surround him. He assures Bowman, saying:
You know that I have the greatest possible enthusiasm for this mission. (24.43)
My onboard memory is quite capable of handling all the mission requirements. (26.23)
Okay, cool, so there's a big talking computer on board. So what?
Well, beneath all that efficiency is a delightfully neurotic, homicidal mess. "Look, Dave, you've got a lot of things to do. I suggest you leave this to me," (26.30) Hal says, by which he means he's going to kill all the people in hibernation. Hal's a sneaky supervillain; he's cunning and deceitful; you can't trust what he says.
And that's what makes him seem so human. Bowman always says what he means; Floyd sometimes lies, but always in the interest of the greater good. Hal has a bit of him that isn't just about the mission. He's got a goal that isn't the goal of the novel; that's all his own. Bowman doesn't even seem to fear death, but Hal does:
Dave…I don't understand why you're doing this to me…I have the greatest enthusiasm for the mission…You are destroying my mind…Don't you understand?...I will become childish…I will become nothing…. (28.47)
The novel for the most part only expresses the emotions of awe and wonder, but with Hal's death, it, for once, manages real sadness.
So why is the crazed computer the only one in the book who seems recognizably human? A big part of it is that the book is about progress—and Hal is the one big blip in the progress. Hal is a technology that doesn't work and messes up; he's the non-human brain that gets homicidal and evil, rather than (like the Star Baby) powerful and beneficent. Hal doesn't strive for the stars; instead, when he's supposed to strive for the stars, he decides it's more important for him to stay alive. He's retrograde in the path of advancement. He's going backwards while the rest of the book is going forwards.
And that makes him human. The book is all about progressing beyond meat and flesh and worries about death and turning into super star people who know all and see all and are, as a result, really boring. Hal feels backwards—and backwards, in this book, is the only way to feel human at all. Hal is not with the program, he does not, in fact, feel enthusiasm for the mission. Unlike all the other aliens in the book, Hal is flawed. And flaws are what make you human.