Of all people, Ridley Scott had all of my votes when it came to pulling incredible performances from actors, and actually improving the acting in his film through close involvement with the actors and a balls-to-the-wall, passionate vision for what he wanted.
Seems like with Prometheus, all of that passion was applied to the visuals, and the actual acting and characters were somewhat of an afterthought outside of representing these huge, obvious, and often revisited 'grand themes.'
I remember watching the extras for Black Hawk Down, and some of the actors talk about random things that Ridley would do to make the performances more geniune, like having random explosions going off around them that they had no clue about. Apparently that was back in the day when he really paid attention to what the performances looked like on screen.
I don't know whether to blame some modern trend or oversight on Scott's part. In the end, though, I think the general audience noticed.