Help keeping the video recording while turning off screen

I have the GS3 and have purchased the 64 gb sandisk microsd card(I receive it today). I will be writing a full review soon since i couldn't find one.

I would like to know how to turn off the screen while continuing to record. Since the 64 gb card will give me about 8 hrs of recording, so they say, i plan on putting it in a corner and have it record everything. Is this possible? if so how? If not, what app can i buy to get this to work? Yes i am making that kind of video.

I am scared of leaving the screen on for so long since i think the heat will mess up my phone in more ways than one.

The spy camera apps dont use the phones video program which is awesome on the gs3.