"Fergie really threw herself into the family life.
"Often Diana would be seen to be the moody one, and Fergie was the good fun, frolicking girl who was a breath of fresh air in the family."
With Prince Charles feeling the heat, he made a six-word comment to Princess Diana which, to this day, has been described as the "most hurtful thing he could have said".
Psychologist Jo Hemmings told the documentary: "There was a point at which Charles said to Diana, 'why can't you be more Fergie?'.
"For someone whose self-esteem is already pretty low, who is feeling very vulnerable, That wasn't just a rebuke.
"That would have been one of the most cutting, hurtful remarks that her husband could have made to her."
Diana and Ferguson were good friends before tension grew - Diana actually set up Fergie with Andrew.
Fergie got on with the royal family very well due to a host of shared interests. Photo / Getty
Diana also tried to get Ferguson into the royal fold by asking if she could be her lady-in-waiting.
But her request was rejected by the royals, saying Ferguson wasn't suitable.
Royal biographer Tom Quinn claims that the Palace was "nervous" of Fergie, who was loud and outspoken.
He told the programme: "It's not a servant and it's not a friend. It's sort of somewhere halfway between. It's a genteel occupation and gives enormous status.
"Diana was persuaded that it was too risky having Sarah Ferguson, she was seen as a loose cannon."
Diana reportedly took on Princess Anne, who had complained about something Ferguson had done.
Diana effectively asked Anne to cut Ferguson some slack, saying "Fergie is good for the family but she's finding it hard to adjust, just as I did."
