The play, directed by Joe Harmston, will begin the second leg of its run at the Thames Street theatre at the end of the month and Susie has committed to another 10 weeks on the show.
Describing her character, socialite Kay Mostyn, Susie told The Guide: "She is phenomenally wealthy, the richest girl in the UK, so that comes with certain personality traits because of her up-bringing.
"She has always had her own way. I think deep down she is a really good person but is not used to being told 'no'. I haven't looked to one person for inspiration to help play the character because people are so different. The play is based in the 1930s and that is a completely different world."
Produced by the Agatha Christie Theatre Company, the production is in its seventh year, but Susie has a been involved since January.
The story, based on Agatha Christie's 1937 novel Death On The Nile, follows honeymooners Simon Mostyn and Kay on a steamer cruise in Egypt, but tragedy strikes when a body is found.
Susie, 31, was working with the Royal National Theatre in 2001 when she was spotted by an agent and her breakthrough came as Chardonnay Lane-Pascoe, in ITV series Footballers' Wives. Appearances in Coronation Street, Hotel Babylon and Doctors followed alongside a successful modelling career.
Film roles have seen her star alongside Gerard Depardieu in La Femme Musketeer, in 2004, and with James Corden and Mathew Horne in Lesbian Vampire Killers in 2009.
On the transition from television and film to theatre, Susie said: "They might as well be two completely different businesses because they are such contrasting jobs. Theatre is live and you have to be prepared to cope with anything that goes wrong. Both are really fun in their own way."
Murder On The Nile also stars Kate O'Mara as Miss Ffoliot-Ffoulkes, Ben Nealon as Simon Mostyn and Mark Wynter as Dr Bessner.
Susie added: "It has become more enjoyable throughout the tour, which is quite a strange thing. I had an option to leave the show after this first run, but I have taken up the option to stay. I had wanted to get some theatre work and it has been a lot of fun, so why stop?"
Murder On The Nile is showing at Theatre Royal Windsor from Tuesday, August 28-Saturday, September 1. Tickets range from £11-£31.50 with concessions. Visit www.theatreroyalwindsor.co.uk or call 01753 853888 to book.
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