haha.. indeed.. movements are the easier way to handle this situation.
but there are many occasion where i tend to stand too far from the target of interest... and because it meant to be candid... cannot be too near as ppl will change their expression differently...
anyway... thanks for all inputs though...
Integrasky - Think about it - "where I stand too far ..." - are you uncomfortable taking the shot in the first place?! If so, you're not even focusing on your subject, but on yourself, and ANY lens you use would yield less than "interesting" results. Also, when you're sort of "across the street" with a long thing sticking out the front of your DSLR, you will certainly attract way more attention as compared with the casual passerby who snaps with a less threatening, "less professional" setup!
As pointed out above, if posed, anything goes, but if you intend taking candids at a "reasonable" perspective, then for head or upper-torso, you'll get along just fine with any of an 85mm, 90mm or 100mm on a typical DSLR (not full-frame). More important it is to not feel self-conscious, to learn how to feel and look natural with your camera, and to respect your subjects! Why, with the very young and the very old, you can get superb shots even with a 30mm lens from across the dinner table! (Not advisable for anybody else not in the above groups, though!!!)
A warning, though - you might actually have more success with your kit lens prefocussed to the intended distance (appropriate aperture, of course), rather than the higher-image-quality lenses in the ranges described above, as poorly focussed shots with these lenses stick out unbelievably obviously!!! We've all done it, unfortunately, and it only reduces with tons of practice and good judgment of distance. The good prime lenses highlight every flaw in equipment AND technique!
Enjoy!