The preparation for the actors playing Colonial Marines included two weeks' training with the S.A.S. (Special Air Service, Britain's elite anti-terrorist force) and reading Robert Heinlein's novel "Starship Troopers" novel. Michael Biehn missed the training, as he was a last-minute replacement.
Apparently the Sublevel 3 / Hive set wasn't scrapped after filming was done. The set was the reused as Axis Chemicals in Batman (1989). When the Batman production crew opened the set, they found it almost completely intact from Aliens.
It's often mis-stated that when Jenette Goldstein heard about an "open call" for a film called Aliens she thought the film's subject matter was immigrants, so she showed up to it dressed like an immigrant. Thus the line in Aliens, "Yeah they said Alien and she thought they said illegal Alien and signed up." However, this is just a hollywood urban fairytale and completely false.
The Alien queen was an entirely new creation Jim Cameron and his team had to design from extrapolation. In the original, H.R. Giger never intended for there to be a queen. The actual rig was accomplished by standing two men back to back in the suit, and each of their arms operated one major extremity and one minor chest extremity apiece. It's worth mentioning that Cameron's queen suit was lost after the filming of Aliens and when it was needed again for Alien: Resurrection (1997), one had to be loaned by a private collector.
In the scene where Ripley is shown the image of her "daughter" who has passed away while she was floating in space, the filmmakers actually gave her an image of Sigourney Weavers mother!!! Apparently Sigourney felt it would help her feel the emotion of the moment, thus turn in a better performance!
The APC was built from an old aircraft tug bought from Heathrow Airport, London, who also supplied a driver, since the four wheel drive, four wheel steering vehicle was so hard to drive! It could make it up to around 35mph.
The "warrior" aliens in Aliens (as distinct from the queen Alien) were a re-design from the original movie. This is most noticeable in the head: whereas in the original film, a translucent resin dome, or sheath covered the majority of the head's upper surface, in the sequel the sheath was jettisoned as it proved very cumbersome to keep clean and scratch-free.
James Cameron found this to be a happy coincidence, as Giger had designed crests to be suggested through the dome, which he thought looked suitably alien when fully exposed.
He was not, however, a fan of Giger's inclusion of a real human skull at the front of the alien head, and so kept a small smooth forehead (which also kept the xenomorph from looking too different between the two films).
The mechanism used to make the face-huggers thrash about in the stasis tubes in the science lab came from one of the "flying Piranhas" in one of Camerons early films "Piranha II: The Spawning" (1981). It took 9 people to make the face-hugger work. One person for each leg and one for the tail.
The special edition DVD includes extra scenes; Newt's parents discovering abandoned alien ship on LV-426, scenes of Ripley discussing her daughter, Hudson bragging about his weaponary, robot sentry guns repelling the first alien raid, Hicks and Ripley exchanging first names. Also included is a scene on LV-426 where a child rides a low-slung tricycle similar to one ridden in The Terminator(1984), also directed by James Cameron.