What you know is a problem for Prometheus. Actually, it's the problem for Prometheus, perhaps unavoidable in any prequel to Alien, but especially poorly managed here.
This is a nasty, dark little room.
--T.E. Lawrence (Peter O'Toole), Lawrence of Arabia, 1962
Early in Prometheus, David (Michael Fassbender) makes his way through the space vessel named Prometheus. He checks a couple of screens, takes a language lesson, rewatches Lawrence of Arabia. He also checks on the humans aboard his ship and, being a synthetic, has special access to their biological systems. This appears the explanation for his ability to read dreams, which he proceeds to do with archeologist Elizabeth Shaw (Noomi Rapace).
She's the reason the Prometheus is on this voyage, in 2093, to a planet two years away from earth, as she's discovered a series of ancient cave paintings in the Isle of Skye with her partner, Charlie (Logan Marshall-Green). “I think they want us to come and find them,” Shaw says. And so: the discoveries attract the attention of a zillionaire named Weyland (Guy Pearce), who funds the mission and also provides David, his personal best synthetic, here assigned to be the crew's primary assistant, overseer, and recorder. It's not a little creepy that David stands over Shaw hyper-sleeping, both impressively hard-bodied and vulnerable in her white-bandage-like underwear. Eavesdropping on her dream, he sees her at a conveniently pivotal moment, as a child asking her doctor dad (Patrick Wilson) to explain death. When her father tells her about heaven, not-exactly-explaining, "It's what I choose to believe," you know pretty much where this little-girl-turned-action-archeologist-hero is headed.
This is a problem for Prometheus. Actually, it's the problem for Prometheus, perhaps unavoidable in any prequel to Alien, but especially poorly managed in this prequel (which is, according to Ridley Scott, not precisely a prequel, but "does have a connection" to the 1979 movie). You know more than everyone in the movie: you know what happened to Ripley, you know where the aliens are going, you know what faces the crew. And so that crew -- Shaw especially -- has to catch up.
This more or less means that Shaw has to become another Ripley, maybe all four Ripleys, in the four Alien films, realizing the horror of what she's leading the team to find. Her education is slow at first, as the film provides not so compelling Big Exposition. To start, a prologue shows one of the space travelers she pursues letting loose its DNA in gigantic rough water that may be near the Isle of Skye. This lets you know that Shaw and Charlie are probably right to call these travelers "the engineers," though you also guess that the ensuing ambition to meet some engineers to ask why they've made people (and later, why they're bent on destroying their creation) is, of course, ill advised.
Your knowledge makes the outcome of Prometheus' mission a foregone conclusion, but still, the movie spends a long time proffering magisterial views of the ship and planet (recalling Alien, but also 2001 and Blade Runner, among others) and introducing some members of the 17-person crew (in some instances, particularly Kate Dickie's, the barely-there characterizations are exceedingly frustrating). The role players include the Dallas-like captain Janek (Idris Elba) and corporate minder Vickers (Charlize Theron), first seen in her luxurious holodecky suite doing I'm-a-tough-girl pushups and ordering David around. It's soon apparent that she's trying desperately to stay forever young like David (and, notably, like Queen Ravenna) and also that she's competing with him for Weyland's favor.
This matters not only because it creates a sibling-like weirdness, but also because their tense relationship raises more parent-child relation questions, as well as the ever vexing but here sadly unexplored overlap between humans and machines. This overlap is proposed in David's eerie affect (he won't pass as human like Ash) and seeming sympathies, and again, in Vickers' incessantly robotic affect: she's got a cold blond hairstyle, awesome body, and superior attitude, but never quite aware of the full extent of the mission. She also only pops into the action when she's needed to gin up conflict (this is a consistent scripting problem, as a series of individual episodes deliver to your expectations, but very awkwardly). So, Vickers softened for a moment by her complicated (but mostly unseen) relationship with Janek, gives the nominal novice Shaw a hard time, and eventually takes up Ripley's role of refusing to let the ground team back inside the ship, an inclination immediately supported by the fact that one team member grimaces and groans and appears to mutate before everyone's eyes, bearing in him an alien that you know about even if he doesn’t.
Vickers is then essentially dismissed from the action until she's needed for some melodrama versus David. He is aware of the mission (a point you'd surmise because of his Ash-like perception of human bodies as vessels, but also because the script includes a couple of clunky scenes to hammer home this point). David's choices -- especially regarding Shaw -- are erratic, again apparently to serve up plot frictions. Their exchanges are arch and sometimes witty, suggesting that Shaw knows something she can't but also must, as she develops in a single mission Ripley's weird intimacy with the alien and, more portentously, with the corporate mind that means to use the alien.
Here that use is less vividly commercial than before (or, more accurately for the franchise's timeline, will be), but dressing it up in grand notions of universal designs and questions of purpose and meaning doesn't mean the immediate incarnations are any less brutal than in other Alien movies. That brutality has its greatest impact on Shaw, whose personal mission is a mix of religious questions initiated by the dad business, marked by the cross she wears on her neck, and brought (far too obviously!) to crisis when she has sex with Charlie (unlike Ripley, she doesn't have to wait decades for it).
Shaw's Riplification is accelerated by what we see and she doesn’t... though she interprets fast-evolving events as if she does see her teammates' encounters with alien hatchings, wherein jittery geologist (Sean Harris) and nerdy botanist (Rafe Spall) face off with the franchise's signature vagina-penis meldings. Awkward crosscutting between the guys and Shaw (who is engaged in a frankly incredible battle with an alien) stitch together a plot you know before it happens. As Shaw catches up, she also starts voicing for her (rapidly declining number of) teammates what must be done, even as the corporate entity pushes back, holding on to her first hope, that the engineers will provide answers.
And here again the movie runs up against the problem of you. You know, because you've seen Alien, that the answers are not so compelling as the questions, and you know no answers are forthcoming anyway. As hard as Prometheus works to pose vast human-conditiony questions and cast its askers between pre- and post- events, it ends up looking more mundane than expansive. This not because you know the answers to such questions, but because you've seen them asked in more provocative and inventive fashion -- in the very movies cited by this one.