The actor was cast as the Ramsay Street favourite earlier this year after James Sorensen bowed out from the role to pursue a career in the army.
Speaking to Holy Soap, Mullally confessed that the recast has led to some changes in the way the character behaves on screen.
He explained: "We are two different people, James is his own person, so naturally how we feel about situations and people aren't going to be the same in every circumstance.
"One of the writers said to me in a meeting the other day that the changeover between James and me can be best described as James playing Declan as very strong, macho. He liked to get into things and was very proactive.
"The new Declan is more grown up, quite mature and savvy. He thinks with his head and knows what he wants and doesn't have to use brute force to get it."
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