When setting up a haunted house, a theme can be your greatest ally in crafting the scare. By insuring that all of the frightening elements have something in common, the actors do not end up by confusing the guests but can instead build on each element into a complete storyline.
A theme does not have to be complicated and can, for example, be built around props or costumes that the group already has. For example, if the actors are primarily drawn from the Society for Creative Anachronism, your theme could be that of a Medieval Torture House, or the Tower of London with all of its attendant ghosts. Drawing on common costume elements available today, one could create a house based on Movie Moster Horror, with Freddy Kreuger, Béla Lugosi's Dracula, and Elvira in attendance. Or if the location is a creepy Victorian-era house, one could, well, do Victorian Horror.
When I was in college, I took part in a haunted house run by a pseudo-Louisiana company called the Bayou Brewing Company. Naturally, the haunted house had swamp-themed elements in it; however, even though it was very well-designed, it did not have a coherent theme. This became a problem later on when the designer left, because the company gentleman in charge did not have the same sense of drama that the designer had; he allowed the various different elements to blend, which not only sapped the tension but caused confusion - the Aliens birth scene does not, for example, go very well with the creepy swamp guy.
If you are running on a tight budget and can only do a haunted house with the different elements on hand, make sure that you spend a lot of time crafting the narrative. I call it a narrative because it is exactly that: a storyline crafted by the actors and the guests together.
Next up: Narrative