When people talk about stupid characters in Prometheus, a lot of it is taken directly from the characters contradicting themselves. Milburne runs away from a dead alien and sticks his hands into the face of a live, hissing alien. Fifield has the maps and leads the way until the second he doesn't.
Ripley puts Newt in a room she thinks is safe away from the front lines so she can get some rest while the rest of them keep working. That's not out of character. We never see her establishing some "everyone have eyes on everyone at all times" rule or something.