There are a lot of questions about ISO on the Mark II. Here are some answers.
When you shoot, do you set your ISO to 160 when outdoors or 320 with a ND filter over it?
I almost always shoot at 160 outdoors during daytime. In order to get a f/2.8, I will often still use an ND filter. I recommend one which you can purchase from my web site which work great for the 24-70mm f/2.8 Canon lens, if you’d like. (see below) It’s a rotating ND Fader filter, so you can dial in your amount of ND which is really helpful when shooting video so you can maintain a f/2.8 and a constant shutter speed (1/50 for 24P, and 1/60 for 30P).
Would you use 160 ISO for indoor interviews as well? Do you ever go to 320 ISO in an interview indoors?
As for ISO, I would use whatever you need to use. If it’s super bright and you can afford to drop down to the 160 ISO, go for it. If you need a bit more exposure, 320 will work fine. I’d say the vast majority of my interviews that I shoot inside are shot at 320. I’ve had to shoot a few at 640, and the results are still very good.
Basically, I always want to keep my shutter speed at either 1/60 for 30P or 1/50 for 24P and I usually want to shoot wide open (at f/2.8). So then I find the best ISO to deal with the room I am in. I will never change the shutter speed, but I will slightly tweak the f-stop and ISO and the lighting to make the shot work.