The ABC drama will explore the consequences of Castle (Nathan Fillion) and Beckett (Stana Katic) spending the night together, the exec told TV Guide.
"We think it's important to show the fans what this moment means for Beckett and Castle and how it may change and complicate their relationship," he said.
"We're taking a much different approach [and showing] the fun of being in a relationship."
However, Marlowe promised that the Castle / Beckett romance will progress slowly at first.
"After two failed marriages, he worries about screwing it up, and she worries about being wife no. 3," explained the showrunner.
Nathan Fillion recently told Digital Spy that the romance between the show's leads is a new "beginning" for the series.
"What we've actually been witnessing is a long and protracted beginning," he suggested. "We've been waiting for this to finally happen - alright, finally now we can get started."
Castle will return to ABC in late 2012. In the UK, the series airs on Channel 5 and Alibi.
Watch Nathan Fillion discuss Castle with Digital Spy below: