According to this profile, the former Archibald Leach built a facade called Cary Grant and basically lived behind it throughout his Hollywood career. Does filmmaker Robert Trachtenberg provide a window into his subject's soul? Not really. But the documentary does give us a good idea of the work and calculation that went into the creation and maintenance of the Grant persona.
More important, the abundant clips in A Class Apart recall an incomparable star who was equally adept as a screwball comedian and a suave romancer. Costar Eva Marie Saint says here that Grant charged fans 25 cents per autograph during the making of North by Northwest. A little greedy of him, but I'd gladly pay.