The Queen Mother told a young Queen Elizabeth to be "unapologetically" in charge.
Lady Penn - a former lady in waiting for the royals - recalls hearing the 91-year-old monarch's mother tell her daughter to always walk in to a door through the centre of a doorway to show she is in charge.
Speaking in a new Channel 5 documentary, Lady Penn revealed: "The Queen Mother told the Queen when she was very young to be brave. I think the Queen probably felt walking into a room full of people was rather daunting. So she said to her, 'what you want to do, when you walk into a room, walk through the middle of the door.' And I think by that she meant, don't sort of go in apologetically. You walk through as if 'I'm in charge.'"
Queen Elizabeth recently marked her Sapphire Anniversary, 65 years on the throne, but she didn't do much celebrating as it only serves as a "reminder" of her father King George VI's death at the age of 56.
Royal historian Robert Lacey said: "She's naturally shy and is the opposite of showy. And, of course, the anniversaries of the accession remind her of her father's premature death.
"There is only so many of these landmark dates that people will want to mark. I'm sure she would rather people were asking why isn't it being celebrated rather than, 'Oh no, not another royal anniversary.' She always plays safe - and that is a good instinct.
"The Queen is always wary of the commercial aspects, and doesn't want to appear 'on the make' - though she doesn't mind if charities use anniversaries to raise money. She's got a well-developed sense of not wanting to exploit these royal anniversaries."