The only thing scarier than what appears onscreen in "The Slasher" is perhaps the bizarre undercurrent of frightening episodes that plagued the cast and crew in real-life during its making. Throughout the shoot, there was a feeling of lurking evil, as if some sort of malevolent presence hung over the proceedings.
On the first day, director Jim Haggerty had his back turned to three cast members as he set a camera angle. Suddenly, the three cast members started asking what he was talking about. When Haggerty turned back to tell them he hadn't said anything, all three claimed to have heard a loud but whispering voice coming from his direction exclaiming, 'Kill, kill, kill,' over and over. All three were certain they'd heard it, but Haggerty repeatedly denied saying anything and the two crew members facing Haggerty when this was supposed to have happened backed up Haggerty on having said nothing.
After it appeared this was resolved, Haggerty turned his back once again to resume setting the shot. Again the cast members began yelling, thinking Haggerty was playing a prank on them. When Haggerty denied again that he'd said anything and the crew members agreed they began to extrapolate that the crew members were in on the gag.
By now, the shot was set. Haggerty called action and just as the actors were about to commence, they heard it again, "Kill, kill, kill," in a whispery voice getting louder and louder. Only this time Haggerty was facing them.
Clearly it was not Haggerty nor any of the crew members. Haggerty asked them why they hadn't begun acting yet, and they told him it was because of the voice. But neither Haggerty nor the crew members could hear it. The cast members walked around behind Haggerty and the crew members, but they couldn't find the source of the voices.
Finally, although it gave them the creeps, they acted the scene while hearing the voices. And after the scene - a brutal killing - all three claim the voices had stopped, almost immediately after the victim in the scene 'died'. It was an eerie moment that was not forgotten throughout the entire shoot.
In another scene, one of the actors talks into a pay-phone. The rest of the conversation would be shot at a later date and later intercut. The actor was simply doing his own lines on the phone. After a few bad takes of his first line, he suddenly ran through the first line flawlessly, paused for an appropriate amount of time, went through the second line, paused again, went on to his third, right up to the end of the scene without a mistake. Afterwards, Haggerty congratulated him on really great takes. He said it was much easier once Haggerty had the voice on the phone reading him the corresponding lines of dialogue to jog his memory.
Only Haggerty had nobody on the other end of the line. It was a public pay-phone. Nobody had dialed. And yet, the actor heard someone acting the lines on the phone.
In another scene, a woman takes her clothes off in front of a window. In the scene, she's being watched from outside, but the shot of the person watching from outside was to be shot on a different day. The actor playing the watcher was not even at the location at the time. Afterwards, she had commented that she didn't know that the watcher was going to actually be at the window at the time of shooting. When told that nobody was there, she claimed she saw the Slasher in full costume at the window. Even though the mask was sitting right in the next room she said she clearly saw someone outside wearing it during the scene. The crew went out to look and found nobody anywhere.
But the weirdest was one really brutal murder scene that was shot late in the shoot. It was brutal and bloody. After it was over, Haggerty congratulated the actors on a job well done and sent them to get the blood cleaned off. They said nothing and went to the bathroom to clean up. After they returned, they didn't speak very much to anyone. After Haggerty discussed a few things with the crew, he began discussing the next scene with the actors. They asked when they would be shooting the murder scene. When a dismayed Haggerty told them they had already shot it ten minutes ago, they told him he was crazy. After some back and forth arguing, Haggerty showed them the video playback of the scene.
Not only did both actors not remember the scene (even while watching it), they both began to sweat profusely and shake. One of them actually went to the bathroom to be sick. It wasn't that they found it so visually repulsive, just that they claimed when seeing it, there was feeling of convulsion shoot through their stomachs. To this day, both cast members have no memory of the scene and have trouble watching it.