Getting back to Lambert, her reaction to being confronted by the creature was a natural one that fit the character. Freezing the way she did, backed into a corner with no space to move, was a hopeless situation for her.
It’s interesting in the way each character reacts to their (final) encounter with the Alien, fits with earlier moments in the film.
Logically (within the film), they behave according to what is revealed about their personalities:
Brett is seen stunned (perhaps frozen) as the infant emerges from Kane, and later, the same response to facing the adult before being attacked. His buddy, Parker - prone to heroics - a reckless character flaw that gets him killed.
The only genuine surprise (characterwise) being Ash, but even here there are clues as to his inhuman nature throughout the film.