Alien and Aliens are, to my mind, almost perfect movies within their respective genres, and this near perfection is nowhere more evident than in their first half hours.
Half an hour into Alien is right before the face hugger hugs John Hurt's face, that is to say exactly when the establishing of characters and situation ends and the horrible things happening begins. Consider what the viewer knows at this point: we know these people are workers, just out there to do a a job and are only investigating the alien transmission because they were ordered to; given the choice they all would have climbed back into their cryo-tubes and gone home. We know they are professional and capable; the landing on the planet was difficult but handled without drama, the repair of the ship the same. We know they are very, very, isolated.
We know the captain, Dallas, is laconic and unflappable, we know Lambert is not, we know Kane is enthusiastic, we know Ash is a bit strange, we know Parker and Brett have chips on their shoulder about being 'below deck' staff. We know Ripley in particular is especially capable. If we are at all familiar with 1970s movies we know this is a hell of a cast. We emphatically do not know which of them, if any, will survive.
When the shit hits the fan, as it is about to do, all their reactions make sense, all their deaths matter.
Half an hour into Aliens is just as the dropship is leaving the Sulaco, which is when the introductions of characters is done and the building of tension really begins. Cameron's film will make us wait nearly as long again before we get any actual action but this is the point when most of the cast start wearing helmets and identical uniforms, and if the viewer has not learned to tell them apart by now they have no hope in the future.
The viewer has though; Vasquez, Apone, and Hudson are all colourful characters who have stood out on the strength of their dialogue (and their names have been repeated more than once). Bishop gets one of the more memorable introductions in all of cinema. Hicks has been a quiet but important presence throughout and his clear admiration of Ripley's demonstration of her power-loader driving skills has made us notice him. Gormon is about to have a couple of lines of dialogue which will perfectly sum up exactly what we need to know about him: "How many drops is this for you lieutenant?" / "Thirty eight... simulated. "How many combat drops?" Cameron is the most incredibly functional scriptwriter. There isn't an ounce of poetry in him but can he ever tell a story efficiently.
What do we know? We know Ripley's motivations in detail. We know we don't trust Carter Burke. We know Bishop seems alright. We know the marines are overconfident, they've been on 'bug hunts' before and consider it beneath them. None of them take Ripley's briefing seriously. We also know they are capable, well drilled, and (Gormon aside) experienced. By the time they are suited up and all their hardware has been displayed it is hard to imagine them coming off second best.
We probably don't know who half of them are, but that half is going to be wiped out soon anyway, the survivors, we are already invested in. When the action does finally come, and Ripley grabs the controls of APC while Gormon struggles to get a hold of himself, it is intensely thrilling because that character work has been done.
Alien: Romulus is not a perfect movie by any stretch. There is a lot to like in it: Cailee Spaeny's and David Jonsson's performances are excellent. The sets are outstanding. The action is well put together. The creature effects fantastic. It adds colour to life in the Weyland Yutani colonies and makes a game attempt to tie together Ridley Scott's quasi-religious Prometheus wackiness with the lore from the earlier films. There are missteps too, the logic of the plot does not bare close scrutiny (who was vacuum packing all the face huggers? and how? and why? and HOW?), the continuous callbacks to the previous films are distracting, and when there was only one alien it was a lot scarier than when it turned out there were many.
None of that would matter so much if it had done better in the first half hour.
I'm not sure exactly where the half-hour point lands but I guess it is as they're first exploring the space station, shortly after Kay has admitted she is pregnant, maybe when they first encounter the half android and a completely naive viewer, were there such a person, would finally know for certain that something very bad had happened there.
What do we know? We know Rain and Andy's relationship in detail, we know Rain has been abandoned on the colony by her parents and the company have screwed her over and that is why she is taking the risks she is to escape. We have met Tyler, Kay, Navarro and Bjorn and we know Tyler is the boss of the gang and Bjorn is a dick and Navarro is religious and Kay is bland and nice. We know Rain was planning to leave Andy behind but this is confusing, it goes against the grain of everything else we know about her and apparently seems to have been added purely to give Bjorn something to be a dick about - it has no real baring on the rest of the film.
In fact I was confused about quite a lot at this point and not much of what I did know had a great deal of baring on the rest of the film. Do the four kids own the Corbelan or are they stealing it? What is Rain's job? What are any of their jobs? (This matters: who can pilot the ship? who can build a flame thrower out of bits in a pinch? Did they just spend their time on the colony sitting around playing video games?) Who are they to each other? Wikipedia tells me Tyler is Rain's ex-boyfriend but I certainly didn't know that when I was sitting in the cinema - it is indicative of the weird sexlessness of modern cinema that despite being all young and beautiful, none of them get so much as a Corporal Hicks admiring look from any of the others. By process of elimination Bjorn must have been the father of Kay's baby but do they actually like each other? You'd be hard pressed to tell. (And are they not cousins!)
The first half hour matters tremendously in these sort of movies. There isn't going to be much time for character development once people are running away from face-huggers and inching past alien cocoons so it needs to get done early. It matters too. Every death in Alien is a wrench which changes the logic of their situation and the dynamic of the surviving crew. In particular Dallas' early loss propels Ripley into taking de-facto command, and she has to step up because some of the others are losing it (Lambert is all for taking the three-person shuttle and leaving someone behind!) The early decimation of the colonial marines nearly breaks them but Corporal Hick's cool head pulls them through. Every subsequent death is earned, even Gorman, who all but disappears for most of the film, gets a great end.
When Bjorn is trying to save Kay it matters if he loves her! It matters if he knows she is carrying his child! When Tyler and Rain are stuck together with scary corporate Andy it matters why they broke up! It matters if they might get together again! It ought to matter that Rain's childhood companion has turned into a scary corporate robot but she was planning on leaving him behind anyway so, meh! It matters if they've stolen the Corbelan or not (are they on the run now? do they have the option of just going back to the colony?). It matters who can pilot the ship and who cannot, why is Rain a 'space virgin' in the first half hour but competently doing space things in the last half hour.
The annoying thing is it would be so easy to fix. What if they had to leave the colony because Kay was pregnant (it doesn't appear to be any sort of place to bring up a baby). What if they had to steal the ship and we got to meet them doing something, showing off their skills rather than just lounging about. What if Rain initially would not leave Andy behind but he persuaded her to, because it was what was best for her. A few minutes thought finds you all sorts of better options which could have given the subsequent action-packed middle of the film a more sure-footed narrative momentum - and I wouldn't have been sitting in the cinema wondering how you vacuum pack a face-hugger because I would have cared too much about people's faces maybe getting hugged.
You can't hope to make an almost perfect movie, but you can try and learn from them.