I do decorative painting/artwork. I've also done several differnt faux techniques on my walls and friends. I can tell you from experience with crackling small pieces of art and furniture, it is a very tricky (and aggravating) technique. I am not trying to be negative, but am trying to spare you a lot of agony. I would not encourage anyone to do this technique on a wall.
Due to the nature of the process, you only get one shot to get it right. (Base coat of paint, then apply crackle medium, then ONE swipe of top coat. If that top coat doesn't go on perfectly (exact thickness) in one swipe, you have a globby mess. Once it's done, it's done and there's no going back.
As Henry said, if it doesn't turn out, you will have major sanding to do to rid yourself of any problems.
There is a faux painting site I really like: www.fauxlikeapro.com This site has great tips for faux painting on walls. This next link is her page on crackling technique, as you will see, she does not mention using this on walls; only cabinets and smaller items. http://www.fauxlikeapro.com/crac.html