Alien is pretty much "alien life form gets on a ship and crew find they are in jeopardy if they don't get it off". I really feel it's scope was way too large for the story.
As others have noted, this was a movie made specifically to be part of a larger framework - at least a sequel and probably a trilogy.
", but the fact is decent writers and directors making decent movies usually do provide actors with some guidance in this area so said actors can..know...indicate to us, the audience, that their characters do, in fact, care about what they are doing. Good stories, at least to me, focus on some particular plot element.
In most cases, really good stories - and by association really good movies - can be boiled down to one sentence synopses.
Further, these three characters actually behave logically for their characters, so it seems that their development was thought out.
After a space merchant vessel perceives an unknown transmission as a distress call, its landing on the source moon finds one of the crew attacked by a mysterious lifeform, and they soon realize that its life cycle has merely begun.
Ellen Ripley is rescued by a deep salvage team after being in hypersleep for 57 years.
After her last encounter, Ellen Ripley crash-lands on Fiorina 161, a maximum security prison.
When a series of strange and deadly events occur shortly after her arrival, Ripley realizes that she brought along an unwelcome visitor.