LONDON — Prince Charles has not been afraid to speak his mind on topics like historical preservation, climate change and alternative medicine. But as king, should that come to pass, it would be a very different matter, he acknowledged.

“I do realize that it is a separate exercise being sovereign,” the prince said about his activism in a documentary, expected to air on the BBC Thursday night, on the occasion of his 70th birthday next week.

“The idea, somehow, that I’m going to go on in exactly the same way, if I have to succeed, is complete nonsense because the two situations are completely different,” he said, referring to the roles of heir and king.

The interview offers a rare glimpse of what Charles might be like as king, and is perhaps an effort to assuage critics who have worried that he would diverge from British monarchs, who are bound by tradition to reign, not rule, over their subjects. His aides have suggested in the past that he would be more outspoken as king than his mother has been.