Nikon D3100 Video Grainy?

I'm using the camera to record video and the lighting is kin o dark (it needs to be for the scene) but the picture quality is grainy. How would I fix that?

Lower the ISO, open the aperture and light the scene more. You can darken it in post.

Actually the scene has to emulate a dark scene, but you still have to add light to keep the cameras ISO low to prevent noise (grain is an attribute of film)

Here is a video shot indoors (for the most part) and the lighting was used to emulate a darkened bar, but I assure you, just like in Hollywood, the light levels were raised to allow noiseless video

Do NOT set the ISO to automatic, that will always add noise if the camera suspects low light in mostly dark scenes.