Although designed by HR Giger the Alien costume was constructed by the American modeller Carlo Rambaldi and, because Ridley wanted the jaws to drip with saliva, Rambaldi had plumbed a tube into the outfit for this purpose through which liquid could be pumped. When all was ready Ridley came over to Shepperton one evening to test this creation and immediately spotted a problem since the tube trailed behind the actor like a second tail. 'Why the blazes is that pipe coming out of his backside?' (said Ridley in words a little less polite.) 'I can't shoot it like that.' whereupon SFX in the person of Nick Allder stepped in to promise 'We'll fix that tomorrow. Leave it to us.'
Roger went home, dived into the shed and plundered some stuff from his aero modelling days. The next day he fitted a battery pack and radio controlled receiver, wired them to an RC switch and attached a windscreen wiper pump. This was hooked up to a fuel tank, liberated from a model airplane, which has the advantage of continuous flow whatever the angle of operation. The whole contraption was installed inside the horns on the back of the Alien costume and the tank filled with a mixture of glycerin and water.
That evening when Ridley came to review the situation the stuntman, Eddie Powell, was in the costume and suspended on wires from the undercarriage leg of the alien spaceship. He was lowered, the jaws opened and, with just the right amount of sinister viscosity, the radio controlled alien drool oozed forth exactly on cue. A delighted Ridley was heard to mutter 'Thank **** for the Brits.'
It was in this undercarriage leg that the captured crew members were stored after they had been cocooned by the creature. At one point Ridley burned these cocoons with the flame thrower but the resulting footage was considered to be gratuitously gruesome and was not included. (source Alienexperience.com, June 28, 2009)