The appearance of the creatures was kept secret from the cast members until the first scene in which they encounter them was filmed. When the cast were finally filming the scene where the girls encounter the crawlers, the girls were genuinely scared and screamed the building down, running off set and laughing.
As stated in cast commentary on the Special Edition DVD, Shauna Macdonald, Natalie Mendoza and Alex Reid did the entire white water rafting sequence on their own without any stunt doubles or special effects.
At Neil Marshall's insistence, all the people playing the villainous "crawlers" were professional actors rather than stunt men or dancers. He wanted them to cultivate a distinct character for their crawler, although, in the finished film, many crawlers only appear for a few seconds.
The "crawlers" were designed to resemble Nosferatu from the film of the same name. They also had huge white eyes to begin with but this idea was done away with because they looked too silly. It took three and a half hours in makeup to transform an actor into a "crawler." They had to shave off their body hair as well.
This film's poster art is borrowed from a portrait photograph by Philippe Halsman of Salvador Dalí, entitled Salvador Dali In Voluptate Mors. (The photo itself was inspired by surrealist Dali's gouache Female Bodies as a Skull painting) This same imagery was also used for the poster for The Silence of the Lambs (1991).
There is a shot of a "beasty" much sooner than you think. Not long after they start the descent, in the cave that they light up red, when Sarah hears children's laughter as is checking out where it's coming from, look in the background to her left and you'll see a silhouette.
According to the audio commentary on the special edition DVD, there was a exact replica made of Nora-Jane Noone and after the movie was finished, she got to keep the head of the doll and put it in her mother's freezer as a joke.
The actors among the female rock-climbing crew and the actors who played the cave-dwellers were largely kept separate from each other through the shoot (including at lunch), heightening their ability to play animosity against the "other cast".
Watch closely during the scene after the cave-in. While the girls are arguing about how to get out, Sarah is moving her flashlight about the chamber. She shines it on a ledge for a fraction of a second as she is moving the light around, and you can can see the shadow or outline of a crawler. When she passes by the ledge again with the flashlight, the shadow is gone, and you can faintly hear a scurrying sound as it is running away.
The trivia items below may give away important plot points.
During Sarah's imagined escape in the blue 4x4, Neil Marshall gave the drivers orders to do whatever they could to damage the vehicle. He was unaware that Sam McCurdy (Director of Photography) wanted to buy it after the shoot.