I carry an 8 inch (total) fixed blade in my right front pocket, exactly where a folder would be clipped. It is just the right size put the pommel just out of sight inside the pocket, but just below the edge where I can get a hold of it and easily draw it. Turns out it rides very well there (with one exception). It has a leather sheath, so the sheath does not stick to it when drawn. The particular knife has a couple downsides, (lacks decent hilt or finger choil and doens't have a good texture on the handle), but overall it works pretty well. No garment to clear, not obvious, draws cleanly and quietly with no fanfare. I need to improve the hilt/grip business, and would like to put a clip on the handle just like those on a folder to make it more secure -- a friend of mine knocked me on my back in practice, and when I hit the ground, the knife came loose. A clip would fix that, altho not as "covert."
I have been experimenting with some smaller fixed blades in other locations, basically neck chain (dogtag type, easily broken) and boot carry. To me, neither seem good for good clean access. Neck carry requires clearing the garment somehow, plus the chain lets the knife swing around a lot. If I end up on my back or my side, the knife swings over under my armpit, and if on my face, I am laying on it. Lengthening the chain brings it closer to the hem of the shirt which improves access a bit (assuming it is an untucked shirt), but amplifies the swinging problem. If you wear a tucked shirt, then access to a neck knife gets even more cumbersome (especially if it is a tucked polo).
I didn't really think I would like boot carry, but it is growing on me. I put it inside left boot at about 4 or 5 o'clock; to draw, I hitch up left pant leg like I am going to sit down, pick up my foot to hand height, and draw with my right hand. Can use opposite leg/hand same way. Works better and smoother than I thought it would, actually. And sitting down, it is accessible. However, it is not as convenient as pocket carry, requires two hands to access quickly...and of course need to wear boots.
Not good for a primary blade, but if for some reason can't carry a blade elsewhere, it works out. I have seen pictures of people clipping or strapping knives to exterior of the boot, either inside or outside leg, but that seems not so hidden, and subject to catching on things.
Lots of folks carry at the waist, either angled or horizontal, IWB or OWB, various places around the belt, with untucked shirt over it. Basically like a gun. It works for them, but of course to keep it hidden you need the cover garment, which then requires clearing, unless you don't mind open carrying a knife. Also, the same friend who knocked me down pointed out that in a close grapple situation, especially on the ground, you end up with blade VERY close to your guts. In force-on-force/ground fighting practice, we have been known to draw our opponent's knives (and guns) and use them against the owner. Things around the waist are very discoverable when grappling. Something to think about.
I like fixed blades because they are simple, solid, and inexpensive, especially compared to a folder with a decent lock (e.g CS Tri-Ad). But I do also carry a Spartan in my other pants pocket. You can fit a lot more folding knife in the same space as a fixed blade.
ETA: There are also arrangements where the knife, in a Kydex sheath, is carried in the pocket or tucked in the belt/waistband, and secured to a belt loop by a short cord, just long enough to allow the knife to be in the pocket or waist. When drawn, the sheath is pulled of the blade by cord. The CRKT Folts Minimalist is one of these (it also comes with a belt clip). A downside is that you need about as much room for hand motion as does drawing a folder that uses the wave-type opening or inertial opening methods.