I already have the 70-200 Canon 2.8L on my 5D MkII
I am looking for a prime lens to use as a second camera on interview video shoots that will give me minmum DOF at a reasonable price.
There are Canon, Sigma, Tamron, Tokina lenses all the way down to F1.2 but with quite large price fluctuations.
Do I need F1.2?
Do I need a Canon ?
What prime is closest to "actual" for a waist to head shot - a 50mm or 85mm?
Would appreciate any suggestions.

I'd think a 50mm 1.4 or 1.8 would fit the bill. Why do you want the minimum DOF if i may ask?

I should've said minimal. I suppose what I'm asking is at what F stop does that out of focus look start to become negligible on a prime. I would think at F4 or 5.6 you're probably not getting much.

On the full frame sensor of the 5DM2, 50mm is what you call "closest to actual". I went with the Sigma 50mm f1.4 because it has the best bokeh in the bunch according to the comparisons I found. It also has a 77mm filter and so it shares filters with my main squeeze EX1R. Here's one review that compares it with Canon 50mm lens:
Sigma AF 50mm f/1.4 EX DG HSM (Canon) - Review / Test Report (http://www.photozone.de/Reviews/392-sigma_50_14_canon)

There's more to blurring the background than a lens' aperture. What you term the "out of focus look" is informally referred to as shallow depth of field or sDOF. It's the depth of the area in focus. The effect on the areas front and back depend on the focal length and the distance of the objects being blurred from the area in focus. Various lens' characteristics cause the blurred areas and the resulting soft blurred effect is called the "bokeh"

For interviews, you are just a few feet from the subject. I found that f1.4 on the 5DM2 was too shallow. It wasn't deep enough for the whole head to be in focus. Get a DOF calculator and you can calculate exactly how depp it is for a given camera, focal length and aperture

Anything in the range of 1.x aperture is like moving a razor blade around in space (I mean that in a good way). I had a 50 1.8, and got the chance to use a 35L and when you get close enough you get ridiculous DOF when wide open (with some NDs obviously, or shooting in darker situations...).

Any prime is going to let you blow the background out very well when you get a bit closer.

I am looking for a prime lens to use as a second camera on interview video shoots that will give me minmum DOF at a reasonable price. [...]
Do I need F1.2?
Do I need a Canon ?
What prime is closest to "actual" for a waist to head shot - a 50mm or 85mm?

I got two old Carl Zeiss f/1.4 -- 50mm and 85mm -- that I'm very happy with. Also, although not as DOF'y as the Zeiss', I recently started using my f/1.8 20mm Sigma, (in fact I'm more happy with that lens than Canon's pricy f/2.4 16-35 L).

Here's a short clip using the 85mm Zeiss at f/1.4:

Test footage from portrait series -- Carl Zeiss 85mm f/1.4 on Vimeo

-- peer

For a head and shoulders shot at 50mm, f/2.8 generally gives enough background blur. Any more than that and you can get focus problems if the person moves more than a few inches.

Having said that, it is always nice to have a lens that is a few stops faster than what you'll usually need. It means you've got some room to move if there is very little light (this is more relevant to events/weddings/run 'n' gun), and it also means that you don't always have to shoot with your lenses wide open, which is when they are softest.

I'd suggest the Sigma 50mm f/1.4 as a suitable lens for your purposes. It's sharper than the similarly priced Canon version, and also has a newer design than the Canon which is starting to show it's age. It's also much cheaper than the f/1.2L which, though an amazing lens, is probably overkill for your needs.

Thanks John - thats good info - I'll definitely look at the Sigma as my prime.

Canon 85mm is f1.2 I believe. Got one, use it at f 2, amazing look with enough d o f. The 35mmcanon prime is great too.

I have a Zeiss ZE 50mm f1.4 for when I want really, really shallow DOF. For interviews I most often use the Canon 70-200 f4. At f4 the background is as soft as I usually want it for interview shoots. Normally for interviews I'm within less than 10 feet of the subject, and the 70mm is wide enough for a good MS, and the 200 is long enough for the ol' eyebrows to mouth shot for emphasis. Usually I want the background recognizable but blurred enough so if somebody walks by in the background and looks at the camera, we don't know they're looking at the camera. There are times I may want to not be able to identify anything in the background, and that's when I use the 50, with enough ND to shoot wide open or nearly so. Usually around a 2.8 on the 5D does that. As mentioned above, with too shallow a DOF you'll lose focus during interviews if the person leans forward or backward too much.

Chris
Some DOP friends of mine and I discussed your problem over a few beers. We've come to this conclusion, you should get this lens, get slightly further back from the subject, and zoom in. It may not be a whole lot less DOF than a 50 1.8, but every little bit helps.
Sigma APO 200 - 500 mm F2.8 lens: Digital Photography Review (http://www.dpreview.com/news/0703/07030805sigma200500mm.asp)
Hope that helps.

And on a useful note, having seen a lot of DSLR footage, there's such a thing as too little DOF. Being in focus is underrated. I'd be hard pressed to shoot any faster than f2.8 if not f4 unless I was jammed for light (and if I was, I'd bring lights).
Just sayin...

Dylan: does that lens come with its own Sherpa guide to carry it?

Thanks Dylan - I'm just $24K short, but live in hope. That is one mighty lens!!
Judging from the comments it seems that a 50mm F2.8 is probably enough for head/shoulders interviews, with reasonable DOF. Under F2 is getting very narrow and over 4 is not enough, broadly speaking.
I read somewhere that 85mm was the closest to an "actual" view as perceived by eye??

I like to use a zoom for interviews. Not to zoom during the tme the subject is talking, but to get tighter or looser between questions without having to move the tripod.

I read somewhere that 85mm was the closest to an "actual" view as perceived by eye??

MY OPINION, for what it's worth, is that on a full frame sensor that somewhere between 35 and 50mm would equate to a "normal" perception of fore/mid/background. 85mm is moving into short telephoto range, which IS actually quite pleasing for "portrait" style shooting...

85mm on a full frame camera is my "go to" interview lens. I find that f/2 to f/4 gives a pleasing DOF. It really depends on how much light you have, how far away the background is, and how much you want to blur it. If the background is a fountain, you might not want to blur it too much. If the background is a messy office, blur the heck out of it! :)