Hindsight is 20/20

I did my usual annual "watch Alien" marathon again. I watch it, watch the deleted scenes and then I watch it with Ridley Scott's commentary.

The characters have come to mean so much to me. I always want them to survive. And I do my usual "if only" routine. If only they had done certain things they would have lived.

It's all 20/20 hindsight. The characters have been called "truck drivers in space." They loaded up their cargo, in this instance it was ore, and spent months in hypersleep while hauling it back to Earth

Captain Dallas was just following Company orders and seemed to have no imagination. He didn't think beyond the regulations that he had to follow. To be fair, humans had probably never encountered any alien species, let alone dangerous ones, before. He was just investigating a signal that might indicate intelligent life.

But if I had been in his place, I would have been a lot more careful. After he leaves the ship to investigate, only THEN does Ripley (who is at least thinking outside the box) put the transmission through for Muthr to decipher it. It's too late for Dallas and the others when Ripley finds out that it is a warning.
If I were Captain I would have done that before I went out to investigate. They did have some time before they arrived at LV 426.

After seeing the long dead space traveler, you think Dallas would have figured that the message was an old one and there was no one to "rescue". He should never have allowed Kane to enter that chamber. There was clearly no one alive to greet them or ask for help.
He disregarded protocols and brought a deadly entity aboard. But after the initial shock he didn't do more than sit around listening to classical music. Again, if I was the Captain, I would have been in sickbay keeping an eye on Kane. Just WHAT was that dark spot on the monitor? He was so quick to believe Ash's explanation. I would have had my engineers run a diagnostic on the auto doc to see if the "spot" was an equipment problem or something the alien put down Kane's throat.
It just seems that Dallas could not be bothered. He brings a hostile organism onboard and then goes to listen to music!
After the shocking chest burster scene, why did the crew underestimate the alien's size? I know they weren't scientists, but they should have understood basic biology. Humans take nine months to gestate and years to grow to adulthood. Dogs and cats gestate faster and are full grown in months. Insects grow to maturity in days.
Didn't it occur to them that a creature who is "born" after 24 hours might grow even more rapidly?

When Brett found the skin which the alien shed, why didn't he run back to the others and say, "I think this thing has grown!!"
It was a whole new ballgame. They were now hunting a larger creature.

It appears that humans had never encountered any alien species before, so the crew was in the dark. But still, their naivete is astounding.