The Alien franchise has gone through a lot of changes over the years, but it turns out that when it came time to design the creatures seen in the third installment, a famous actress was used as inspiration. You wouldn't think that somebody trying to create an updated version of one of the most vicious creatures in science fiction would think that Michelle Pfeiffer was the place to look for inspiration, and yet, it turns out that the actress, specifically her lips, were used to create the new adult alien look. According to Alien 3 director David Fincher...
We did give it Michelle Pfeiffer's lips. That's what they're based on. It always had these little thin lips, and I said to Giger, 'let's make it a woman when it comes right up to Ripley.' So it has these big, luscious collagen lips.
While H.R. Giger designed the original creature in Ridley Scott's Alien he wasn't brought back by James Cameron for Aliens, so when he got the call to for the third installment, he apparently saw it as an opportunity to make improvements on the original creation. According to Monster Legacy, Giger wanted to make the alien more "sensuous" this time around, thus leading him to Michelle Pfeiffer as the inspiration for the new alien lips. I'm sure you can see how the creature above was designed based on the image below.
While we're not entirely certain how "sensual" the creature looks in Alien 3, there are certainly worse places to get inspiration for that in the early 1990's. Michelle Pfeiffer had previously made some very sexy movies like Dangerous Liaisons and The Fabulous Baker Boys and would appear as Catwoman in Batman Returns the same year that Alien 3 was released. If sensual is what you're going for, it's understandable why that particular actress would come to mind.
We're not exactly sure what Michelle Pfeiffer would think of learning she helped inspire the look of one of the most famous monsters in horror film history. On the one hand, it's nice to be recognized, right? But then again, is the all-consuming, acid for blood, alien really something you want to be personally associated with? Also, Alien 3 isn't exactly the most loved entry in the franchise. It's too bad James Cameron didn't bring H.R. Giger back for Aliens, as maybe these same changes could have happened a movie earlier.