The saddest thing about Lee Israel isn’t the constant drinking, or the once-promising career washed up in pieces on the shore, or the flies infesting her filthy apartment, or the fact she’s in her early 50s and her only friend is her 12-year-old cat.

No. The saddest thing of all about Lee is she can’t or won’t acknowledge she could actually be the life of the party if she didn’t walk into every party with armor on and weapons drawn. She’s a brilliant wit with a caustic sense of humor, and if she just let her defenses down and rounded off some of the edges, hanging with her would be GREAT fun.

Melissa McCarthy plays Lee in the artfully constructed and quite wonderful “Can You Ever Forgive Me?”, and while Lee remains a miserable cuss throughout, McCarthy’s performance is such a work of understated perfection and authenticity, we actually enjoy the hell out of every moment we spend in Lee’s company.

In the performance of her career, the inherently likable McCarthy never once tries to endear her character to the audience, never once resorts to any sly winks. She trusts the nomination-worthy script by Nicole Holofcener and Jeff Whitty, and the first-rate direction by Marielle Heller, and the result is great work.

Based on Israel’s memoir, “Can You Ever Forgive Me?” is set in the New York City of the early 1990s, against the backdrop of a winter so bleak and gray, we almost shiver in our seats just soaking it in.

For some 20 years, McCarthy’s Lee has achieved a degree of success and respect with her well-researched, gossip-free magazine profiles and biographies of the likes of Katharine Hepburn, Tallulah Bankhead and Estee Lauder — but as Lee’s agent Marjorie (Jane Curtin, terrific) explains to her irascible client, the market has changed. Salacious, warts-and-all celebrity bios are now all the rage. Nobody wants to read Lee’s straightforward book about Fanny Brice — and nobody in the publishing world, at any level, wants to put up with Lee’s boozing and her insults any more.

Marjorie’s advice to Lee: find another way to make a living.

Down to her last few dollars, unable to even pay for her cat’s medicine, Lee begins forging letters “written” by Dorothy Parker and Noel Coward, among others, and selling them to collectors for cash. (Lee uses typewriters of various vintages, traces signatures and lightly bakes the stationery to achieve an aged effect.) Lee’s such a talented writer, and such a fan of the authors in question, her fakes come across as genuine and believable.

For a while, at least. Even as Lee takes great pains to cover her tracks, we get the feeling it’s only a matter of time before the authorities come knocking on her door. You can only fake it for so long.

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Richard E. Grant excels as the charming but weathered Jack, who maintains the front of the dashing man about town but is clearly deep into a downward spiral, and is just as lost and lonely and isolated as Lee. A longtime acquaintance of Lee’s, Jack becomes her drinking buddy, confidante, something approaching a friend, and eventually her partner in crime. They make for quite the duplicitous, untrustworthy duo — but for a few hours here and there, they come close to letting down their guards and enjoying each other’s company, almost like “regular” people do.

“Can You Ever Forgive Me” is filled with memorable supporting performances, including Anna Deavere Smith as Lee’s ex, Elaine; Ben Falcone as an unscrupulous memorabilia dealer, and Dolly Wells as a lovely and sweet bookshop owner who takes a shine to Lee.

The cinematography is vibrant and feels true to the period — but this is not the romantic New York City of untold movies. The locales and the color tones reflect the broken-dreams worlds of Lee and Jack, in particular.

This isn’t the first time Melissa McCarthy has left her comedic comfort zone to tackle something more layered. At times “Can You Ever Forgive Me?” is actually quite funny and of course McCarthy is great in those scenes — but she’s equally effective in the darkest, most dramatic moments. It’s one of the finest performances of the year.

‘Can You Ever Forgive Me?’

Fox Searchlight presents a film directed by Marielle Heller and written by Nicole Holofcener and Jeff Whitty, based on the book by Lee Israel. Rated R (for language including some sexual references, and brief drug use). Running time: 107 minutes. Opens Friday at local theaters.