"It's not the house. It's Josh." That line is uttered during Insidious: Chapter 2, James Wan's follow-up to his genuinely creepy 2011 hit about a haunted little boy. If it seems familiar, it should: The big payoff line in the previous film was, "It's not the house that is haunted. It's your son." So, the new version is pretty much the same, only slightly less elegant and less impactful.
The same could be said of Chapter 2 in general, which suffers much the same fundamental problem as horror sequels for time immemorial: They go back to wells that just aren't that deep.
Cinematic history is littered with classic horror films' throwaway sequels (and sequels of sequels) that are so bad that they nearly sully the memory of their forebears. The greater the heights attained by the initial installment, the farther the sequel tends to fall. Take two of the greatest horror films of all time: The Exorcist and Rosemary's Baby. Their follow-ups are not only terrible examples of the genre; they may be among the worst films ever made.
Exorcist 2: The Heretic offered up a bunch of science-fiction hokum about synchronizing subconsciouses, a confused jumble of Christian and Assyrian mythology, badly rendered locust plagues, and Richard Burton doing the worst B-movie slumming of his career. Its only redeeming moment is the unintended hilarity of watching Burton yell at a Washington, D.C., metrobus driver for eating a sandwich instead of driving. And all one really needs to know about the Rosemary sequel is that it was a TV movie of the week starring Patty Duke, titled Look What's Happened to Rosemary's Baby.
Those are two of the worst, but the list of other offenders is long and undistinguished. Jaws 2 practically remade the original, providing evidence that telling essentially the same story with the same characters in the same setting doesn't guarantee that you'll have the same success. In Texas Chainsaw Massacre 2, director Tobe Hooper decided the best way to follow up the grim grindhouse realism of the original was to make it into a comedy. Poltergeist 2 offered an expanded glimpse into the netherworld that trapped Carol Anne during the first film, and added in a creepy skeletal old man and bunch of patently offensive stereotypes about Native American mysticism.
Why do these films tend to be so terrible? A few reasons. If it's true that shorter means scarier, then it should come as no surprise that dragging a horror tale out over multiple installments dilutes its power. Where economical slasher flicks and ghost stories hit quickly and efficiently, ending before the viewer can find the invisible strings that good directors use to manipulate their audiences like marionettes, sequels tend to open the curtain and show too much. Mystery and uncertainty fuels frights; complicated mythologies and explanations of backstories that were only ever meant to be veiled references dispel those mysteries and work against the forces that made the first movie suspenseful to begin with.
But those are artistic concerns, which are no match for financial ones. Just try telling a studio exec that there's no more story to be told to follow up a horror flick that made $100 million on a $1.5 million investment--as was the case with Insidious. Those are rates of return that'll give any investor excited goosebumps, so who cares if the sequel induces a similar reaction for audiences or not?
The fact that the industry makes sequels with no motivation other than avarice is hardly news, but horror resists success in that treatment more than any other genre. Part of the issue is that studios treat horror as a genre as an ATM to begin with. The films are cheap to make and provide solid returns even if they're not big hits. When landmark horror films get made, it's often an accident of someone lucking out and throwing some money at a director who happens to have vision that extends beyond profits.
Frankenstein, Night of the Living Dead, Last House on the Left, Halloween, Texas Chainsaw Massacre, Jaws, The Blair Witch Project: These are all films made by relative unknowns at or near the beginning of their careers. To be sure, some of these films had sequels, and not all were bad. Bride of Frankenstein is as much as classic as the original, as is Dawn of the Dead. It's probably worth noting that in both of those cases, the directors of the originals (James Whale and George Romero) stuck around for the follow-ups, which usually isn't the case with the worst sequels.