unless the image is stretched or shrunk.
When you DON'T get the black bars, on a widescreen TV, when your playing something with the 4:3 aspect, then the image is stretched.
without stretching, you get black bars, OR: possibly, sometimes it will "enlarge" the video, so that the edges are covered. but this means that the side that didn't have bars before will now have parts of the video "outside" of it; quite the opposite of the black bars, since your not missing parts of the video. the reason this is used is the preserve the aspect ratio; otherwise, what should be a circle will appear as an oval.
the modes you select on the TV select the stretching mode. but really, it's not that important; with 4:3 on a widescreen television, you will get black bars on the left and right; when playing a widescreen movie on a standard 4:3 television, you have black bars on the top and bottom.
personally- I just live with the black bars. If I don't, I either stretch everything so it's all screwy (everything is stretched vertically) or it get's stretched so parts of the video are no longer visible. neither of which I find desirable.