Back in the olden days when people sent handwritten letters through the mail, Hal Holbrook wrote a beautiful one to me. Actually, it wasn’t all that long ago — Feb. 23, 2004, to be exact. But Holbrook, who died late last month at age 95, was in every sense a gentleman of the old school, in manners, in love of performance, in devotion to his craft.

My favorable review of “Mark Twain Tonight!” at the Warner Theatre — a solo show he had been performing for 50 years — had run in The Post that day. He’d immediately put pen to “Hal Holbrook”-embossed note paper to tell me how delighted he was that I “got it.” (Tip for artists communicating with critics: Tell them that they thoroughly understood what you wanted to convey, and you’ll melt their jaded, overstimulated hearts.)

“I remember the first intelligent and insightful review I got as Twain 47 years ago,” Holbrook wrote, in elegant cursive. “It was in Emporia, Kansas (was it the Gazette? Mr. White’s paper?),” he asked, referring to William Allen White, the influential Emporia newspaper editor of the mid-20th century. “It was the first time someone wrote down what I was trying to do and it meant the world to me because I knew then that I had something.

“It encouraged me to keep going and keep digging. I’ve done that.”

Holbrook was 79 when he wrote that, and he would keep going and digging as Twain for 13 more years, finally retiring the performance in September 2017. It was announced Tuesday that he had died in Beverly Hills on Jan. 23 — when at last, he could put up his feet and share a cigar somewhere in the firmament with the great American novelist and raconteur who had been so good to him for almost his entire acting life.

There were, of course, other peak moments in Holbook’s professional career, most vividly and belovedly — especially at The Washington Post — his portrayal of Watergate informant Deep Throat in the 1976 film “All the President’s Men.” Holbrook raised lurking in shadowy corners of underground garages to a cinematic art: Remember the trace of gravel-voiced contempt he exuded as he spooned out morsels of damning info to Robert Redford’s Bob Woodward? I’ve watched the scene dozens of times, in which Redford’s reporter loses patience with his arrogant, ambivalent source — and imagined an entire play based on that flammable moment.

That was the lightning-bolt excitement a great actor could apply to a supporting role, or what is sometimes attributed wrongly to the work of a “character actor.” (Any part an actor plays is a character!) Holbrook was revered by others of his craft, his touch so finely wrought on a stage or on film that you’d hear his colleagues cite him as an exemplar of the understatement they also sought to embody.

Holbrook’s patrician bearing and seamless technique allowed him, at earlier inflection points in the evolution of American entertainment, not only a distinguished career in the classics but also a certain utility-man quality. That may help to explain why he wasn’t, in the conventional sense, a star.

As noted in “Another Day’s Begun,” Howard Sherman’s new book about “Our Town,” Holbrook played Thornton Wilder’s narrator, the Stage Manager, on three occasions, the last at Hartford Stage in 2007. His eagerness for work led to some lightweight TV assignments over the years on drama and comedy series, including a recurring role on “Designing Women,” opposite his wife, the late Dixie Carter. There were also more substantive portraits, as in the breakthrough 1972 TV movie “That Certain Summer,” about a gay relationship, and a part in the 2007 film “Into the Wild,” which earned Holbrook an Academy Award nomination for best supporting actor.

But it was his Tony-winning turn as Mark Twain that staked Holbrook’s claim on the American imagination. Think of it: a half-century playing the same part. The idea of an actor repeating a role ad infinitum carries some sad connotations: Eugene O’Neill, in “Long Day’s Journey Into Night,” built tragedy on the back of the play’s father, James Tyrone, whose stage career languished after his thousands of times playing “The Count of Monte Cristo.” For Holbrook — who started portraying Twain in his 70s when he wasn’t yet 30 himself — there was no such pitiable dimension.

If Holbrook didn’t invent the monodramatic concept of inhabiting a historical figure on a stage, he certainly popularized it, in a way that made it a revenue stream for any number of other actors over the years. From Julie Harris (“The Belle of Amherst”) to Laurence Fishburne (“Thurgood”), great actors can doubtless trace their own success in the genre to the impact of “Mark Twain Tonight!,” a piece Holbook never stopped refining and revising.

“Holbrook strolled on like an unannounced guest and exited with the same to-hell-with-the-bells-and-whistles casualness,” I wrote of that 2004 performance at the Warner. “Between arrival and departure came 100 minutes of sarcastic wit enveloped in devastating charm, the 1905 equivalent of devilish standup.”

I quote it here only because of the response it elicited from Holbrook. At 79, he was still eager to be relevant.

“Your review today has reaffirmed — reaffirmed — all I hope for the show,” he wrote. “And it will keep me going.”

His work has kept us all going.