In the scene where Dallas is crawling through a duct and Lambert becomes hysterical as she sees the dot representing the xeno approaching his position, what could Lambert know of the appearance of the xeno currently?
I think the audience has gotten a glimpse of the creature at full growth when Brett sees it and is attacked and I think Parker also got a glimpse because he says it was "big as a man" or something -- it had against all expectation increased in size literally 100s of times from the chest-burster.
So all the uber-hysterical Lambert has to go on is what Parker said -- had I missed the Brett scene, I would still have visualized as the small creature I had last seen scurrying away. An admittedly terrible little creature but actually not something threatening to an adult human -- it could be batted away or incinerated.
It seems also that the cattle prod device Brett made assumes small size, right?
So what did the crew know of the creature when Dallas was attacked? I think all they had to go on was a brief description by Parker indicating it was much larger and perhaps he implied it was man-like in appearance?
I think the actors were responding to the big version of the xeno even though they really should not have known much of it -- I wonder if the cast was allowed to see the full-sized monster before the duct scene -- I know that brilliantly the director surprised everyone with the chest-burster scene.