“I felt very insecure and inferior the whole time because his moustache was unbelievable,” he told Graham Norton
The 40-year-old actor, best known for his role as Christian Grey in the Fifty Shades film series, stars in A Haunting In Venice, which sees Agatha Christie’s famous Belgian detective return for another mystery.
Sir Kenneth also played Poirot in the previous films Murder On The Orient Express in 2017 and Death On The Nile in 2022, based on Christie’s novels.
Dornan told The Graham Norton Show: “It was very different doing scenes with (Sir Kenneth).
“I felt very insecure and inferior the whole time because his moustache was unbelievable, the best in film history as Poirot, while mine was a wee attempt at one.”
Dornan, from Northern Ireland, also starred in Belfast, Sir Kenneth’s autobiographical drama written and directed by the veteran actor.
Set during the Troubles, it is partly based on the 62-year-old actor, writer and director’s own experiences as a boy and earned him a best original screenplay Oscar last year.
Dornan was asked about being reunited with Belfast co-star Jude Hill in the Poirot film and said: “I love that boy so much. Even though he is now a star he is still the sweetest kid.”
The Tourist star was also joined on the BBC chat show by actor James Norton, Oscar-nominated Hotel Rwanda actress Sophie Okonedo, Derry Girls actress Siobhan McSweeney, and singer-songwriter Lewis Capaldi.
Norton said it was a “huge relief” that people are enjoying the third and final series of Happy Valley.
When the six-episode BBC One drama, created and written by Sally Wainwright, returned on New Year’s Day it garnered five-star reviews from newspaper critics across the board.
The series follows Sarah Lancashire’s character Sergeant Catherine Cawood on the trail of murderer and sex offender Tommy Lee Royce, played by Norton.
Norton said: “(Tommy) is a despicable monster, but he became really enticing, a sort of weird distant friend that you love seeing.
“I loved playing him and I miss him and the show.”
The Graham Norton Show airs on BBC One and BBC iPlayer on Friday at 10.40pm.