Visible crew/equipment: In the Reactor Room scene, where the Marines first encounter the aliens, there's a part where Dietrich is grabbed from behind and accidentally ignites Frost with her flamethrower, causing him to fall down the stairwell. As he's falling, if you look, you can see part of a leg and a cable belonging to one of the technicians on the set. (01:14:05)
Visible crew/equipment: Throughout the sequence when the female pilot is preparing to drop the ship with Ripley and the marines on down to LV426, you can see a crewmember in her reflective sunglasses.
Visible crew/equipment: In the scene at the colony when a large vehicle approaches the camera, the reflection of someone's face can be seen showing the true scale of the miniature.
Visible crew/equipment: A wire can be see on the Alien Queen's left leg as she descends from the ship just after ripping Bishop in half.
Visible crew/equipment: When an alien rises out of the water to kidnap Newt, you can see two wires lifting its tail out of the water.
Visible crew/equipment: A wire is visibly attached to the queen's tail as it bursts out from Bishops' chest. (Slow-mo helps but is not necessary.)
Visible crew/equipment: In the scene where Newt and Ripley are trapped in a room with two facehuggers, Newt traps one with a table, and you can briefly see the operator's hand manipulating the legs.
Visible crew/equipment: Just after Ripley drops the first flare (on her way back to rescue Newt), the shadow of a crewmember is cast on Ripley's back.
Visible crew/equipment: When Ripley is first shown in the loader after asking Apone if she can help, after the loaders claws grab the crate and she swings the crate around and asks "where you want it?" you can see tubes trailing off to the left of the loader that don't connect to anything in frame. The tubes are on the right side of the frame just under the crate in the loader's arms.
Visible crew/equipment: During the first attack scene in the hive, a marine gets killed and as he falls between the stairwells, his flamethrower goes off. Just as he falls down into the hold, you can see a crew member's foot moving backwards out of the shot.
Answer: It really was all down to James Cameron having already written the script and proving himself capable of directing with 'The Terminator.' It was just a quicker, easier, and almost certainly cheaper decision to let him direct his own script rather than get someone else, even Ridley Scott. While the producers had wanted to make an 'Alien' sequel almost immediately, at the time the head of 20th Century Fox didn't want to pursue it fearing it would be seen as an obvious cash-in and flop. When a new executive at the studio came in a couple years later, the project was put back on track, and I believe Cameron was the first to be approached to write the script.
TonyPH