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Remember that scene on the bridge, from the outside, where we see Lambert walking past the bay window?
Here’s a clue as to how they did the shot, which you can see somewhere around 20:13 to 20:17 (I’m using the 20th Anniversary ALIEN DVD as a guide here).
See the clapperboard behind the window panes?
Pre-recorded footage was displayed behind the bridge window miniature, creating a very believable illusion that a crewman (in this case, Lambert) was walking by on her reluctant way to join Captain Dallas and Kane on the expedition to find the unidentified signal.
Oh, the charm of practical special effects…
(from The Prop Gallery website.)
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