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/ Source: TODAY
By Eun Kyung Kim

Her grandson, Prince Harry, calls her a tough act to follow but in a new documentary about England's monarch, Queen Elizabeth also is described as “very generous” by Duchess Kate, who speaks of the monarch's guidance to the next generation of royals.

"Our Queen at 90,” the documentary that aired Sunday night in the United Kingdom, provides rare interviews with family members. Among them, Kate, the Duchess of Cambridge, who says her husband's grandmother has been very gracious in helping with her transition into her royal role.

Kate, the Duchess of Cambridge, with Queen Elizabeth.Phil Noble / Reuters

“She has been very generous,” said Kate, the wife of Prince William. “I feel she's been there as a gentle guidance really for me. Everyone teases me in the family that I spend far too long chatting so I think I have still got to learn a little bit more and pick up a few more tips.”

RELATED: Prince Harry honors 'remarkable' grandmother Queen Elizabeth

Britain's Queen Elizabeth II (R) distributes Maundy money during the Royal Maundy Service at St George's Chapel in Windsor, west of London, on March 24, 2016. Maundy Thursday is the Christian holy day falling on the Thursday before Easter. It commemorates the Maundy and Last Supper of Jesus Christ with the Apostles. The Queen commemorates Maundy by offering 'alms' to senior citizens - retired pensioners recommended by clergy and ministers of all denominations, in recognition of service to the Church and to the local Community. During the Royal Maundy Service The Queen distributes the Maundy money to 90 men and 90 women one for each of The Queen's 90 years. RICHARD POHLE / AFP - Getty Images

Duchess Kate also described what a doting great grandmother the queen has been to 2-year-old Prince George and Princess Charlotte, who turns one in May. Kate said Queen Elizabeth is just “Gan-Gan” to the kids, who get small gifts whenever they stay overnight with her for a visit.

Prince George talks to Queen Elizabeth outside the christening of Princess Charlotte of Cambridge last July.Chris Jackson / Getty Images

The documentary also features Prince Harry paying tribute to his grandmother, admitting he struggles with the "huge expectation" he has felt growing up in the long-reigning monarch's shadow.

"Trying to follow in her footsteps is a huge, huge deal. How does one do that? Where do you even start?" he said.

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The documentary was released as a tribute ahead of Queen Elizabeth's 90th birthday on April 21. It will also feature President Obama, who praises the queen for being an “inspiration to people around the world.” He is scheduled to meet with the monarch for lunch at Windsor Castle later this month.

Follow TODAY.com writer Eun Kyung Kim on Twitter.