One of the things that I have always found striking about 2001 A Space Odyssey is the almost mechanical acting of the human characters, especially the radio operator. I'm pretty sure it's a conscious choice on part of Kubrick, but what is it supposed to symbolize?
I have watched all of Stanley Kubrick's movies. He wants you to know that he's in control of the pace of the movie. You'll be fed each line when he's ready to feed it to you.
All of Kubrick's movies tend to have a lot of white space, empty space where no one is doing or saying anything; they are merely existing. Listen to the non-human pace of Nicole Kidman's character (in Eyes Wide Shut). You are hanging on every word.
Kubrick creates tension through his use of pace. The actors aren't being stoic. They are moving and speaking at a real life pace, not the pace of normal movies.
From my perspective, it's partly to contrast HAL's childlike behavior and to also show human evolution from apes to logical beings.
One of the things I found interesting about the entire sequence in the Discovery is a sort of reversing: Bowman and Poole behave coldly, don't give in to emotions and seem--at least emotionally--always in control of the situation. HAL, on the other hand, is the one that cracks under pressure and panicks.
So, of all the crew, HAL is the real human character.