Every film has a lot of people that contribute thier efforts towards the end result and occasionally a little thing can make a huge difference. The original Alien is one of my favourite films ever and the creature is just a clear win. It's often said that one of the reasons Alien is so suspenseful on the first watch is that you don't really get a good look at the Alien and so it forces your imagination to fill in the blanks. On top of this, the big danger was revealing the creature to be a man-in-a-suit, something that everybody in the produciton was keen to avoid.
However, that might not have always been the intention. There's a popular phrase in film-making that any film is actually three diffferent films, the one you write, the one you shoot and the one you edit. Whilst it's often good practice to shoot things "just in case", you can usually tell if a scene was intended to be in the film or not by the amount of extra attention it's recieved. For example, if the shot is a deleted scene but it's been graded and has nice sound design then it was probably in the film until relatively late before it got cut. Alien has three credited editors. Terry Rawlings, Peter Weatherley and David Crowder for the Directors Cut.
With that in mind, have a look at the following deleted scene from Alien from near the end of the film when it's Lambert's turn to get a forehead full of metal teeth. You can see the creature really clearly here and whilst I like what they did with the tail unravelling, the whole scuttle-across-the-floor move looks terrible. It's safe to say that this was shot as an possible option rather than anything else as it's not even got any sound synched up. I do find it interesting seeing all the things that were tried though.