The three controls of exposure are aperture (diameter of opening), shutter speed (how long light is allowed in to the sensor) and ISO setting (how sensitive to light the sensor is set). Video is a VERY different beast for effects of shutter on the resultant files than still shooting. Which is why when shooting video yea, you're supposed to keep the shutter set for a specific "feel" throughout your shooting.

Google for demonstrations and explanations of cinematic/video such as this: http://www.red.com/learn/red-101/shutter-angle-tutorial

Note that when you see a bar below one of the examples with " 45* 90* 180* 360* " on it, hover over each (or click) and you can see what the above situation would look like with different effective shutter speeds.

A "360" shutter is a shutter-speed the same as your frame-rate or "fps". If you are shooting at 30 frames per second and your shutter is at 1/30th, you've got a "360" shutter. A 180* shutter is double your fps, and is one that our brains are sorta used to because it's the main one we've seen in movies over the years. 90* and 45* shutter "angles" are not used very often, and mostly for effect. Or ... from inexperience. Or at times, because you have to because of light as it exists just to get the shot.

Keep it in shutter priority in the creative movie mode, but lock exposure ( and focus) with the AE/FL button or assign just the AE lock to that button. Have a read of the manual.

If there's some way you can avoid using an auto-exposure mode while filming, I suggest doing so. I've tested some shutter-priority auto with my GH3, and got what I realistically expected ... which is to say occasionally spot-on exposure, but if say I panned or something bright or dark moved through the scene ... there'd be that weird mental effect of sudden changes in lightness to everything ... so I went back to manually setting exposure.

Takes almost no time to do, and gives accurate results every time ...