The actor's character Phil Mitchell embarks on a desperate cover-up next week after his troubled son Ben (Joshua Pascoe) kills Heather during a furious argument at her flat.
Even though Heather was the best friend of his partner Shirley Carter (Linda Henry), Phil decides that he must put his own flesh and blood first - doing everything he can to help Ben and hiding the truth from Shirley.
McFadden told Inside Soap: "I would say that this is one of the best storylines I've ever been involved in - and I've been here a long time.
"When we first got the scripts, I said to Josh, 'There's a scale of one to 10 when you get a storyline here, and this is a 10. So roll up your sleeves, because you've got a lot of work coming your way. It could be great if we get it right'. And I think we are getting it right."
The 52-year-old said that he would be "sad" if Phil and Shirley split up as the story progresses.
"Phil and Shirley are great together - they're a match made in heaven," he explained. "But they split up and then they get back together again - that see-sawing always goes on, and if you want to have a career in soap, I think you have to embrace that rather than bemoan it."
McFadden also confirmed that he is planning to keep in touch with Cheryl Fergison, who played Heather, as they became good friends on set.
EastEnders airs Heather's final episode on Monday, March 19 at 8pm on BBC One.
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Watch Phil's EastEnders character trail in the video below: