With a mixture of humility and great ambition, former Imagine TV prexy David Nevins is eagerly stepping into what he calls “one of the great gigs in the television business” as he prepares to succeed Robert Greenblatt as entertainment prexy of Showtime Networks.

“This is one of the only opportunities that could take me out of the producing stuff that I’ve really enjoyed doing,” Nevins told Daily Variety. “Clearly, Showtime has had enormous success and consistency the last several seasons. But I don’t think that they’re completely full yet. I feel like there’s still opportunity. I’m humble but creatively ambitious. I’ve spent my career looking for shows that really push the medium forward.”

The Nevins-Greenblatt transition, first reported Thursday by Variety.com, was made official on Monday with an announcement from Showtime Networks chairman and CEO Matthew Blank. Greenblatt leaves big shoes to fill after a strong, seven-year run that saw the pay cabler emerge from HBO’s shadow to be a player in its own right in original programming.

“The reason David is here is that he has developed some of my favorite shows,” Blank said, citing “24,” “Arrested Development” and “Friday Night Lights” — shows that Nevins shepherded during his eight years as prexy of the TV wing of Brian Grazer and Ron Howard’s Imagine Entertainment.

Nevins will formally move to the CBS-owned pay cabler later this summer after a transition period for both Imagine and Showtime. Greenblatt’s contract runs through mid-July. He quietly let it be known several months ago to Blank and CBS Corp. boss Leslie Moonves that he was ready to move on after his contract expires next month.

Showtime already has several high-profile new originals on deck to bow during the next year, starting Aug. 16 with the Laura Linney starrer “The Big C.” The dysfunctional family drama “Shameless,” led by William H. Macy, will launch in the first quarter of next year, as will comedy “Episodes” starring Matt LeBlanc. Costume drama “The Borgias,” toplined by Jeremy Irons, is targeted for a second quarter bow.

“With a tremendous batch of new stuff coming over the next year, David has the luxury of getting involved with those shows and looking around for the best new material that would take us forward in a way that is tune with his sensibility,” Blank said. “One of the luxuries of the premium TV business is that we don’t have a development season per se. David doesn’t have to be ready to go with X number of pilots by any particular date.”

Blank emphasized his respect and appreciation for Greenblatt’s track record in keeping Showtime well stocked with distinctive skeins like “Dexter,” “Weeds,” “The Tudors,” “Californication,” “United States of Tara” and “Nurse Jackie.” He called Greenblatt “one of the finest executives in the business.”

Nevins said he would cast a wide net for new development, but would also make it a point “to make sure that the shows we have on the air and in development are well taken care of.” Gary Levine, Showtime exec veep of original programming, will be key in maintaining that consistency, Nevins said, noting that he’s known Levine for many years.

“I’m not going to be the guy who tries to fix what isn’t broken,” Nevins said. “But I plan to be fairly aggressive in terms of being open for business. The timetable is always a little slower in pay cable compared to what I’m used to in the broadcast environment. I plan to pick up the tempo a little bit.”

Prior to joining Imagine in 2002, Nevins was exec veep of programming at Fox Broadcasting, where he spearheaded comedy and drama development. Before that he was a programming exec at NBC working on series including “Will and Grace,” “The West Wing,” “Law and Order: SVU” and “Homicide: Life on the Street.”