If you're shooting with a consumer camera, the first thing is to make sure it is completely in manual mode, because what you want to do is (probably) shoot at 0 db, shoot at a wide aperture (maybe f/2.8, for example) and then light the attic to an overall underexposed level so that the flashlight reads as the brightest source of light in the room. If you didn't lock in the manual settings, the camera would attempt to expose for the underexposed setting and start boosting the gain, etc.
It helps to have some justification for the light in the attic, like some moonlight leaking through ventilation grills or something. You don't have to be too logical about it but this can help you justify some cold edge lighting, back lighting, etc. This will help define the shape of the actor, since the flashlight will generally be shining in front of the actor and not lighting the actor.
As for the actor's face, there are two approaches: one tends to be a very soft dim fill light, cold light the moonlight backlight, sometimes from one side rather than frontal, just enough to read the facial expressions without looking "lit". The other approach is to suggest that the flashlight is boucing off of what's in front of the actor, reflecting some light back into his face. Sometimes if the flashlight is bright enough, you can do this for real by placing some white cards and boards off-camera for him to pan the flashlight into, bouncing light back into his face. If that's not enough, you can time his movements with the flashlight with someone panning a small light onto the bounce card to create more light reflected back into his face. But it should still be an underexposed light.
Generally for a moody scene, you want to think about graphic images in terms of contrast of light and dark -- so as he moves around, at some points he could be a silhouette against a light pattern on a back wall or against a wash of backlight coming through smoke (smoke in general helps with seeing the flashlight beam) and at other times, he may be lit frontally by the glow of the flashlight but framed against a dark background. Dark against bright alternating with bright against dark.
