Imagine an acerbic love child miraculously spawned by Addison DeWitt of “All About Eve” and Waldo Lydecker of “Laura,” with John Simon serving as midwife, and you will be prepared for Jimmy Erskine, the viciously witty and mercilessly demanding title character played with utterly delicious flamboyance by Ian McKellen in “The Critic.” Directed by Anand Tucker (“Shopgirl”) and written by Patrick Marber (“Notes on a Scandal”), the film is a heady brew of period thriller, compelling melodrama and jet-black comedy, and the second most remarkable thing about it is how seamlessly these diverse elements gel.

Even more remarkable, however, is McKellen’s multifaceted portrayal of the man aptly known as “The Monster,” both behind his back and to his face, in the film’s world of 1930s London theater.

Erskine takes unseemly delight in savagely shredding the productions (and performances) he finds lacking, and the physical appearances of actors he deems unattractive.