The police are unable to access the dressing room/murder scene because the door has been bolted and padlocked. The officer then takes an ax not to the bolt or lock, but to a flimsy panel, after which the door just swings open.
Further into the film it is explained the actual door might be locked and sealed but the door frame is hinged which is why the dressing room was easily accessed by Mrs Main to put flowers there in memory of her husband.
When back in the theatre, the character Keith Gilbert approaches the camera (fade-to-black) but can be seen clearly looking right into it to judge his approach. The film has many instances of this fourth wall being broken.