The role that changed Hugh Bonneville’s life didn’t arrive in a “Eureka!” moment, and he doesn’t treat it that way in his charming memoir, “Playing Under the Piano.”

“Downton Abbey” made itself known to him as a make-conversation chat with his director, Julian Fellowes, on the set of an earlier movie they made called “From Time to Time.”

“You writing anything else at the moment?”

As recounted in the forward to “Playing Under the Piano,” Fellowes mentioned a few projects, and this “Gosford Park” “great house,” its owners and its staff saga he was about ready to pitch. He’d had great success scripting his first “Upstairs/Downstairs” melodrama, “Gosford,” for Robert Altman. And even though the genre was stale and dead at the time, Fellowes had a hunch. He did think Bonneville, one of a legion of British character actors appreciated by fans but not all that famous, was “too young to play a dad.”

Bonneville’s reply would change his life.

“I am a dad. Of three girls, marriageable age.”

“Downton” drifts through “Playing Under the Piano,” summoned up here and there to make a point about why one avoids eating what’s served to you in a scene (many takes, from different many angles, they have to match in continuity, making a LOT of brownies disappear from “Notting Hill”) or how you never know if what you’re doing is going to click with the public, much less become a global phenomenon.

And Bonneville lets the dressing-for-dinner soap opera bookend his book with a lovely remembrance of Maggie Smith’s last scene, the last day of shooting the last film and even the New York press junket, savoring something that he never actually comes out and says “changed my life.”

It’s a brisk, florid biography in the standard actor’s life mold — “Hugh Boo Boo” childhood, memories of literally playing under a piano, first crushes, first roles, first time he figures out his character actor’s “stocky” niche, first time he is so “in the moment” that he makes something spontaneous and fun happen onstage during the run of a play.

The picture that emerges is of an affable chap who recognizes his privilege — son of a doctor who doted on him, whom he doted on in turn, prep schools, etc. — and the career he’s made out of that.

The anecdotes aren’t sizzlers, as he’s not retired and his former and possibly future colleagues aren’t dead and still in the position of possibly hiring him again. Well, he takes one good shot at director Mike Newell. And everybody knows Christoph Waltz is a “wanker.”

But there’s no “dishing” about Elizabeth McGovern or the Divas of “Downton” — just a note on Smith’s “reputation” — a warm note on Judi Dench‘s acting generosity and a lighthearted look at Julia Roberts, offhandedly throwing her Big Star weight around during “Notting Hill” to the betterment of the film and the benefit of her much lower-billed co-stars (ensuring Bonneville and others were flown to the NYC premiere), gratitude to Kenneth Branagh for hiring the Laertes in his stage “Hamlet” (Hugh) for a bit part in his “Frankenstein,” memories of films like “Iris” (he played the Jim Broadbent character as a young man, naturally) and “Burke and Hare.”

And the childhood recollections are occasionally amusing, but conventionally upper middle class, a long list of the semi-obscure corners of England where he grew up, schooled and summered.

The pursuit of an acting career, after entertaining thoughts of the law and the pulpit at Cambridge, makes for a fun account — meeting Olivier at a dinner party his parents dragged him to, failing to get his foot in any door, shortening his “Hugh Richard Bonniwell Williams” name to something even more posh. He tells cute, self effacing near disaster stories about auditions and recreates a National Youth Theatre/ National Theatre/RSC and Stratford world that he learned his craft and came of age in.

I tracked him down for a chat when the first “Paddington” bear picture came out, and found him much more “Notting” and less Lord Grantham, a fellow who recognizes the good fortune that moved him from lower billings to leads, the generosity of his “Downton” benefactor Fellowes when the chance came his way to join a George Clooney project (“Monuments Men”).

As for the career, movies like the recent thriller “I Came By,” which had him at his most villainous, suggest he has a few surprises in him.

On the whole, he comes off as you’d hope, disarming and not terribly self-serious, sentimental and enthusiastic about the work, if more laid back “British” about it than your average American “Actor’s Studio” alum or emulator.

“Playing Under the Piano: From Downton to Darkest Peru.” By Hugh Bonneville. Other Press. 372 pages, with index. $28.99.