But to say that she disappeared into these roles is not quite accurate. Rather, she used the particularities of these disparate characters to reveal some essential facet of herself, an ineffable but unmistakable Streepness. In one of the few skeptical assessments of this elusive quality, Pauline Kael, reviewing “Sophie’s Choice,” suggested that Ms. Streep was too controlling, too calculated, to create characters of full and spontaneous humanity. “It could be that in her zeal to be an honest actress,” Ms. Kael speculated, “she allows nothing to escape her conception of a performance.”

Often, though, this thoroughness was a virtue, bringing to vaguely conceived, unevenly executed films (like “Sophie’s Choice,” and also “Out of Africa”) a discipline and clarity they otherwise would have lacked. Not a greater dimension of realism, exactly  Ms. Kael was on to something in intuiting a crucial element of premeditation in Ms. Streep’s performance  but a precision that made the characters interesting even as it called attention to the virtuosity of the actress portraying them.

Given the bias that reflexively elevates heavy over light when it comes to screen acting, it may seem perverse to argue that the much-lauded dramatic roles of the ’80s are preparation for the comic flowering that began at the end of that decade with movies like “She-Devil” and “Postcards From the Edge.” Ms. Streep hardly abandoned drama, or the bravura explorations of challenging accents and hairstyles. (See her syllable-perfect turn as the bored, expatriate Italian farmwife in the perpetually underrated “Bridges of Madison County.”) But she began to reveal a playful, mischievous side, an anarchic impulse that, joined to her formidable timing and technique, has blossomed in the past 10 years or so.

You can hardly say, looking at “The Devil Wears Prada” or “Julie & Julia,” that her gift for deep mimicry has diminished. Or that, in “Doubt,” her commitment to dramatic intensity has waned. In her portrayal of Julia Child, the posture, the voice, the bubbly blend of ruthless drive and irrepressible joie de vivre are almost uncanny. But these days impersonation is the province of every aspiring biopic star. Ms. Streep’s Julia Child is never anything other than a performance, a fusion of two strong personalities rather than the absorption by one into another. You never forget that you are watching Meryl Streep inhabiting a version of Julia Child, and instead of distracting you from the truth of Julia, this awareness is what enables you to understand her. It is our familiarity with Ms. Streep that makes her exploration of a more private Julia Child both credible and exhilarating. Similarly in her frequent forays into the world of cinematic statecraft, she can be an entirely convincing senator or cabinet secretary while still being, in every syllable and flourish, Meryl Streep.

She is also, evidently, enjoying herself, and the delight in watching Ms. Streep these days  whether as a spacey folk singer in “A Prairie Home Companion,” a romantically confused divorcee in “It’s Complicated,” or even that steely Irish-American street cop of a nun in “Doubt”  comes partly from a sense of shared pleasure. All of the old discipline is there, but she seems to be having more fun.