Washington is well-acquainted with Harry Belafonte and vice versa. He has protested here and antagonized lawmakers, presidents and their advisers for years; he exploited his connections when he deemed it urgent and necessary; he participated in political triumphs and official dinners and inaugural balls; he has been feted, honored, awarded. He is one of this town’s original visiting celebvocates.

So it might be tempting to channel-surf past “Sing Your Song,” Susanne Rostock’s thorough and moving biographical documentary airing Monday night on HBO, as it exults in Belafonte’s success as singer, actor, activist and humanitarian.

Even if you think you know plenty about the man — and his Zelig-like ability to have appeared front and center at everything from the election of John F. Kennedy to key civil rights marches in the South to the recording of “We Are the World” and the end of apartheid — you will still learn something about the difference between a celebrity who just talks and one who is also willing to walk.

“Sing Your Song” presents an impressive parade of footage from Belafonte’s concerts and TV appearances, allowing his skill as an entertainer to dovetail with his increasing devotion to protest and charity.

Much of his early television work— especially his variety special that aired in 1959 — incorporated an array of diverse musical forms and a racially mixed selection of performers that was controversial then and would in some ways still be a radical departure for network TV today. Not because of who or what is shown (or because Petula Clark wrapped her arm around his during a TV performance in 1968, causing network conniptions), but because today’s TV audiences wouldn’t know what to do with Belafonte’s commitment to cultural depth and artistic intensity. We’re too busy paying rapt attention to amateur singing and dancing.

Belafonte, 84, describes the major moments in his life — “This nuance, this rhythm of events” — that shaped his beliefs. He speaks in a reflective, gentlemanly croak; the voice no longer has that manly calypso lilt, but it is captivating all the same.

“Sing Your Song” is broad and complete, but like most biographical documentaries of legendary performers that we’ve seen of late, it is also hagiographic. Somewhere in the credits, it seems the subject (or one of the subject’s family members), is involved as a producer. Here it’s Belafonte’s daughter. In Martin Scorsese’s recent George Harrison documentary, it was Harrison’s widow. In doing press for a recent HBO documentary about Gloria Steinem, the network’s head of documentaries said the intent of that film wasn’t a biography but an effort to introduce a younger generation to “St. Gloria.”

This doesn’t necessarily make the films worse or less trustworthy, especially when such arrangements can guarantee an intimate level of access to archives and innermost thoughts. But it does tend to coat things in a gloss of friendliness by allowing the subjects a certain measure of control. A viewer can just tell.

Thus, “Sing Your Song” notes but never probes deeply certain aspects of Belafonte’s ego or some of his less uplifting political stances. Two marriages failed — also noted, without elaboration, and his children speak vaguely of how his long absences affected them. Intriguingly, we learn of Belafonte’s apparent victimization by a psychoanalyst who persuaded him to entrust his business affairs to her husband, who turned out to be a government informant who spied on lefty celebs. But we don’t learn enough about how he became susceptible to them. It is addressed and shelved because, like most broadly biographical documentaries, “Sing Your Song” has much ground to cover.

And it covers it all. Watching Belafonte involve himself — even now — in the lost ’60s art of consciousness-raising makes even more clear how disconnected and politically lobotomized today’s celebrities are. Although they are largely regarded as know-nothing liberals by their critics on the far right, our stars are mostly drawn to “safe” causes, to which they can glamorously ally themselves with a few carefully modulated statements and appearances. They eschew domestic politics and class concerns (have you seen many A-listers at Occupy Wall Street?) in favor of speaking out “against” cancer or furrowing their brows over world hunger. Harry Belafonte could still march circles around them.

Sing Your Song

(105 minutes) airs Monday at 10 p.m. on HBO.