The following is inspired and paraphrased from Lookalike: 101 by Greg Thompson. When Thompson talks about getting the look right as a celebrity impersonator he stresses that there is more than just the natural look, makeup and wardrobe.
Mannerisms, speech and expressions are also top on the chart. A good look combined with these three are absolutely necessary to be a successful celebrity impersonator or lookalike.
Celebrity examples Thompson gives are Austin Powers, Jack Sparrow, Buddy Holly, Joan Rivers, Cher, Jack Nicholson, Robert DeNiro and Whoopi.
Some of the necessities with these characters, at least in regard to voice, is to perfect the sound, plus the use of sound bites. Learn each sound bite, and...
"Get it down pat!"
In other words, one has to learn the British accents for Powers and Sparrow including sound bite "Yeah, baby" for Powers, "Can we talk?" for Rivers, and some celebs, like Cher, do not necessarily use sound bites, but rather have a distinct way of speaking. So again, Thompson stresses, whatever each celeb has or does not have, "Get it down pat!"
All this, in combination with expressions, use of hands, stance and reactions, makes for a successful impersonation. None of this comes off well, though, unless the talent is in full costume, not in street clothes impersonating for the fun of it, or:
"Otherwise, you might come across weird... trust me on this." - Greg Thompson