In the novel, you first see the man-ape Moon-Watcher disposing of the body of his dad, and feeling very little about it. After Moon-Watcher dumps his dad for the jackals:
He never thought of his father again. (1.10)
And that more or less sums up Moon-Watcher. He's more interesting to spend time with than Bowman, because man-apes are more interesting than bureaucrats. But fundamentally, there's not much to Moon-Watcher. You're told he's "almost a giant" (1.6) among his people, and that he's a "genius." (4.16) But as far as inner life goes, he doesn't have much of one. He tries to find food; he tries to survive. And then the slab lands and pokes him to hope for a better life, which means using tools to get more food and killing the rival man-apes when he has the chance. But as far as what makes Moon-Watcher particularly Moon-Watcher, or how he's different than the other man-apes (other than being bigger and stronger)—you don't learn anything about that.
You could argue that man-apes don't really have an inner-life; that one man-ape is much like the other. But, that rather ignores the fact that David Bowman and Heywood Floyd don't have much in the way of inner lives either. Moon-Watcher is just like them—he might as well be them. His eyes are set on the stars, and those eyes set on the stars are about all that the book cares about. Human or man-ape, all that matters is that you're going to evolve into something else. What you do in the meantime while you're waiting to evolve doesn't matter, and as far as the novel is concerned, doesn't even really exist. The novel says:
Now he was master of the world, and he was not quite sure what to do next. (6.11)
But really it's the novel itself that doesn't know what to do next. Mastery and power and progress are the whole deal; beyond that, the novel has little interest in describing what people (or man-apes) might want, or feel.