As the ISO value is dynamic in video recording, is there a way to somehow force the camera to record the whole video in one fixed ISO ? Camera: Nikon D3s
You can just turn off the AUTO ISO feature and set the ISO manually. This way it won't change throughout your recording, and this will also produce more consistent lighting, unless the lighting conditions change (i.e. your scene involves turning the lights on/off, opening/closing a window or something like that). Instructions on how to do that can be found in your camera's manual.
A quick google search yielded the following result:
By default the D3s won't give you much control of video capture and you'll have to unlock the manual video control by using a secret method. Don't worry, nothing will happen to your $5,000+ camera!
All you have to do is set your Live View-setting to "Tripod Mode", enter Live View using the Lv-button on the back of the camera and then press the OK-button on the left side of the LCD. That's it. You've now gained full manual control of video on your D3s. Congratulations!
And here's an excerpt from a more extensive guide to the camera's shooting menu:
ISO sensitivity settings
This duplicates the ISO button. I use ISO 200.
ISO sensitivity auto control
This is where we set the AUTO ISO feature ON or OFF. Auto ISO magically bumps up the ISO as the light gets weaker, saving you a lot of time since you no longer need to watch your lighting or shutter speeds. Set this and just shoot, from daylight to moonlight.