In the film, which arrives in cinemas tomorrow (September 15), Reilly plays Rowena Drake, a retired opera singer who holds a séance during All Hallows' Eve in post-World War II Venice to communicate with the spirit of her late daughter.
"It feels a little edgier, a little darker, and it has an intensity to it that I personally love," the actress teased in a newly-released featurette.
"This story is just full of questions and it's up to the audience to decide for themselves [what the answers are]."
Branagh, who once again reprises his role as master detective Hercule Poirot, added: "[Rowena] is such a complex character that Kelly brings such delicacy to".
The threequel has already set a new Rotten Tomatoes record, having earned the highest critics' score of the entire franchise.
"This is a fantastic development of the character Hercule Poirot, as well as the Agatha Christie franchise," Branagh previously said of the film, which also stars Michelle Yeoh, Jamie Dornan, Mayor of Kingstown's Emma Laird and Call My Agent's Camille Cottin.
"Based on a complex, little-known tale of mystery set at Halloween in a pictorially ravishing city, it is an amazing opportunity for us, as filmmakers, and we are relishing the chance to deliver something truly spine-chilling for our loyal movie audiences."
A Haunting in Venice is released in cinemas on September 15.
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