For many years, he has unburdened himself of his views on architecture, education, the environment, homeopathy and countless other subjects.
He has shared his thoughts on faith, on duty and on his respect for tradition. But on kingship and the kind of monarch he sees himself as, he has largely been silent.
In many ways, of course, it has been an entirely commendable position to adopt. Given the longevity of the Queen, to have done otherwise would have been not just discourteous to his mother, but potentially dangerous.
However, the absence of any clear vision of what life under King Charles III will be like has led to a situation where his public interventions – notably those infamous 'black spider' memos to ministers – have seen him cast as a meddling monarch in waiting.
Now, with one eye on the future and from the comfortable position of almost reaching his 70th – his birthday is next Wednesday – he has chosen to deal with these vital issues for the first time.
Questioned in a BBC television documentary about those accusations and about his future role, his answers are both surprising and revealing.
The film also sheds an intriguing light on his relationship with his sons William and Harry, and on his grandchildren. 'When he's there he's brilliant,' says William pointedly. 'But we need him there as much as possible.' What emerges from this 60-minute documentary is the most compelling – and intimate – picture of the Prince of Wales since the controversial Jonathan Dimbleby programme almost a quarter of a century ago.
But while that film marked his anguish over his affair with Camilla, and hastened the final break-up of his marriage to Princess Diana, this one is likely to be remembered for him finally setting out how he will operate as king.
Clearly irked by questions about his lobbying and the accusation of meddling, the prince says he is proud to have worried about inner cities and to have tackled other issues.
Pressed about whether such campaigning will continue once he comes to the throne, he replies: 'I'm not that stupid.'
Then, with one penetrating aside, he says: 'I think it's vital to remember there's only room for one sovereign at a time, not two. But the idea somehow that I'm going to go on exactly the same way, if I have to succeed, is complete nonsense.'
Here, in a handful of words we get to the heart of Prince Charles. On the one hand he is explaining why he has said nothing about his future role.
There is the deep respect and, yes, love, for his 92-year-old mother, and a refusal to take anything for granted. 'If I have to succeed' is a telling phrase in itself.
But there is another message here, too. And this one is directed at Prince William, who will himself succeed to Prince of Wales when the time comes: who knows, it might even be a warning.
Though even the most enthusiastic of William's supporters would admit that the 36-year-old heir in line is more likely to model himself on his inscrutable grandmother than on his outspoken father.
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Indeed, in a candid aside William, who has not always had an easy relationship with Charles, says: 'I want to be my own man and take my own style.'
As for the future, Charles is unequivocal. Without mentioning the word king, he says: 'Clearly I won't be able to do the same things I've done as heir.'
He cites Shakespeare as illustration: 'You only have to look at Henry V or Henry IV Part I and 2 to see the change that can take place, because if you become the sovereign you play the role in the way that it is expected.'
Many will no doubt be reassured by such positivity from the prince, especially those alarmed that – on occasions – he does blunder into the political arena.
The Queen, of course, has been nothing short of remarkable in observing the convention that royals do not express their views on any area of public life.
What this film seems to suggest is that, as the moment of his elevation to the crown grows closer, he is taking these symbolic positions of the monarch more seriously and more thoughtfully.
The picture that also emerges is one of contentment. Only once, in a project which spanned a year in the prince's life did I – a long-time observer of Charles's princely temperament – get a glimpse of the crotchety prince.
It comes during a clip filmed a Birkhall, the prince's Scottish retreat which he inherited from the Queen Mother.
In the gardens there he has planted an arboretum which he refers to as 'George's wood', because he began planting around the time of the birth of his first grandson.
He is walking with Camilla in the gardens when she begins to reminisce about the prince's long-dead Jack Russell, Tigger. The dog was part of his life at the time his marriage to Diana was breaking up and, despite his second wife's endeavours, he clearly did not want to discuss the pet.
All the same, the nervous tics, the fidgeting with cuffs and never knowing what to do with his hands, were largely absent from this warmest of portraits.
The relationship with his sons, which has been troubled, seems to have much improved. They are generous in their praise of their father – especially Harry, who talks of the workaholic prince falling asleep at his desk.
He mentions his gratitude to his father for walking Meghan down the aisle at his wedding in May. And in a nod to his American wife, not used to late dining, he suggests that in the future his father might eat dinner earlier in the evening.
Camilla, of course, is ever present, saying on air the kind of supportive messages that have been such an integral part of their decades-long relationship.
Significantly, she discloses that Charles does not fret about the future. Does his destiny weigh heavily on his shoulders, she is asked at one point? 'No,' she responds, 'his destiny will come, he's always known it's going to come. I don't think it does weigh on his shoulders at all.'
There is, naturally, no contribution from the Queen. But wouldn't it have been fascinating to have heard from Prince Philip?
This was, in many ways, a clearing of the decks interview, not unlike the decision he took on another birthday a few years ago to correct some long-repeated misconceptions about him.
He said it was untrue that he had seven boiled eggs presented to him for breakfast each day, and explained that Camilla still has her own home because she likes to spend time there with her children and grandchildren.
This exercise is far more serious – and risky. Will his openness about how he will reign be believed? If he can succeed in persuading critics of that, then it will have been a gamble worth taking.