Downton Abbey star Allen Leech has revealed the cheeky comment he made to the late Dame Maggie Smith when they were working together and her surprising reaction.
The Irish actor worked with Dame Maggie, who died last year aged 89, for more than a decade on the period drama which has spawned six series and three movies.
Ahead of the last film instalment Downton Abbey: The Grand Finale, Mr Leech, who plays chauffeur turned estate manager Tom Branson, has discussed what it was like sharing the set with the famously waspish dame.
Leech will tell the Ryan Tubridy show on Virgin Radio UK: 'She was the best craic if you were on the right side of her, and I thankfully was.
'But if you gave her too much, if you were too reverential to her, she almost hopped on it.
'She wanted to just be one of the ensemble.'
On one occasion while working on season two of the historical series, Dame Maggie greeted him telling him: 'I'm glad you're back, Smiler.'
Leech said: 'I was like, "oh, thanks very much". And I thought, this is your chance now not to be too reverential.
'So as she passed and she was in this big, you know, what they call that big bustle dress of the period.
'And I just said, "Maggie?" And she went, "What?" And I said, "Your arse looks great in that."
But rather than chastise him for his for his x-rated comments, she was amused by his cheeky irreverence.
He added: 'Immediately, she did that laugh and then I was in, you know, because I was one of the gang.'
The actor also said that Dame Maggie was similar to her acidic character Violet Crawley, the Dowager Duchess of Grantham, whose withering remarks became legendary on the show.
He said: 'She was [like her character]. And she used to say things after she would say something that you knew was like absolute gold, but dangerous.'
It comes after Downton Abbey fans broke down in tears as the late Dame Maggie was honoured in an emotional trailer for the last ever film.
The new film will hit cinemas on September 12 and the first look showed a glimpse of matriarch Lady Violet Crawley, played by the late Maggie Smith, in a painted portrait.
'She'd turn and she'd go, "Oh, I'm wicked. I'm wicked." She knew herself. She was great fun.'
It comes after Downton Abbey fans broke down in tears as the late Dame Maggie Smith was honoured in the emotional trailer for the last ever film.
The tale of the Granthams was first introduced to viewers on ITV back in 2010.
Since then, the franchise has evolved into six seasons as well as Christmas specials and two films.
Now 15 years later, the story is set to come to a close and the trailer for the third and final film has been released by Universal Pictures.
Downton Abbey: The Grand Finale is set in the 1930s and follows the Crawley family and their staff.
Fans will see their favourite characters return to screens as they tackle taking care of the Yorkshire County estate and the changes to come.
It will see the return of beloved stars Hugh Bonneville, Elizabeth McGovern, Michelle Dockery, Laura Carmichael, Jim Carter, Phyllis Logan, Robert James-Collier, Joanne Froggatt, Allen Leech, Penelope Wilton, Lesley Nicol, Michael Fox, Raquel Cassidy also feature.
The final film also features new additions Paul Giamatti, Joely Richardson, Alessandro Nivola and Simon Russell Beale.
The new film will hit cinemas on September 12 and the first look showed a glimpse of matriarch Lady Violet Crawley, played by the late Maggie Smith, in a painted portrait.
Maggie passed away last September and the sweet nod to her through the painting made one fan emotional.
One penned on YouTube: 'This made me weep :(, ohh Maggie Smith how missed you will be..'
While others were left in tears over it being the end of the long-standing tale and took to X: 'Buying tissues now.'
'Don't want to say goodbye,' another commented followed by a crying emoji and a heart.
While someone else said: 'CRYING UNCONTROLLABLY AT A TRAILER!'
The full interview will be broadcast on Virgin Radio UK on Friday from 10am.