The 33-year-old, who plays Aidensfield policeman Joe Mason, is to take on the role of a mysterious character called Alistair who quickly befriends paramedic Polly Emmerson (Sophia di Martino).
Speaking on yesterday's Paul O'Grady Show, he said: "I play a very dodgy guy, quite damaged from living on the streets, who is not as nice as he first seems."
At the beginning of the year, ITV axed Heartbeat and its spinoff The Royal, having stockpiled a sufficient number of episodes to see transmission through to 2010. Filming for the final episode of Heartbeat took place last week.
Speaking of the show's axe, McFadden said: "It's sad, you know. I had a great time on that show. Some of the girls got a bit emotional last week when we filmed the last one. I mean it's 17 years. It became a big part of my life. I will miss the people.
"We were all joking that for the show to continue we had to find a way of introducing judges and a panel. That's what you seem to need for TV nowadays. It is still so popular with the fans. You'd get 300 - 400 showing up when you are filming."
He added: "We had a thank you party for the village where we film, last week. They were so sweet. It was great. They all came. We had to say thank you."
Filming is currently underway for series 24 of Casualty, which is due to begin transmitting in the autumn.
> 'Heartbeat' villagers petition against axe