The concept that the Alien takes on characteristics of its host was how it was originally conceived for the first film (Ridley Scott spoke about it during production). It's always been official
Just to highlight that there are
actually several interpretations to this piece of history, and which one chooses to believe is a somewhat up to individual preference.
Sir Ridley has in interviews contemporary to the production indeed mused around the speculation that "hey — seeing that Giger's design turned out to be more or less anthropomorphic — what if the creature actually takes on properties of its host?" or words to similar effect.
For instance, in Starlog's Official Movie Magazine from 1979 we can read:
FACT: There was much more to the Alien than met the eye. Since Ridley Scott used a painting by Giger as the basis for the adult Alien, he had to work backward to create the first two phases. During this process, the director came to understand the organism completely. "The nasty one," he says, "the thing that sprung out of the egg — the 'perambulatory penis' as we used to call it — is the father. All it does is plant the seed. And the next generation takes on characteristics of whatever form it landed on." This means that the ALIEN may not always be a biped! It could conceivably be a combination of the original Face Hugger and whatever host it uses!
To me, this would indicate that the Alien is not made bipedal in the movie because
it takes on characteristics from its host. Instead, Sir Ridley is speculating that it might take on characteristics from its host because
Giger's design is vaguely anthropomorphic, and Sir Ridley "works backward" from Giger's design. The interviewer also uses words like "may" and "conceivably"; hardly giving an impression that his subject had spoken in terms of definite intent.
Interpreting this instead as indicative of that this was the guiding intention behind the concept all along — let alone "proof" of it — would, to my mind at least, be reading a bit too much into it.
To wit, I have yet to see any contemporary material unequivocally stating that Giger was explicitly commissioned to design a creature that "carried traits of its host" in some way. Rather the other way around, if anything.
And this being the case, I find categorical statements to the effect that the Alien taking on characteristics of its host always was integral to the film's "original concept" ... very difficult to justify.
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