With Showtime having crossed the 20 million barrier in subscribers for the first time, network entertainment prexy David Nevins said the mandate is to maintain the channel’s programming sensibility without becoming complacent.

“There’s no substitute for staying on the cutting edge,” Nevins said at the Television Critics Assn. press tour Thursday. “A healthy network is in a constant state of renewal and reinvention.”

Showtime’s audience base grew by 2 million in the past year to reach 20.5 million. In 2011, “Shameless” was the network’s most-watched drama premiere in seven years, “Borgias” then broke that record three months later, and hopes exist that upcoming Claire Danes starrer “Homeland,” given an Oct. 2 launch following the sixth-season premiere of long-running hit “Dexter,” will itself set a new mark.

“?’Shameless’ started us in a new direction,” Nevins said. “?’Shameless’ is bigger; it has more scope to it, (but) I don’t think there’s been radical shifts in the sensibility (except) a little more emphasis on comedy in the comedies.

“I think we’re still trying to find the subversive characters, but I think we’re busting out of a certain formula that has defined a number of our recent shows. I think we’re a collection of those shows. I don’t think we need to be one narrowly defined programming brand other than being sophisticated and being adult. … I think we have a number of shows, all of which have to be the favorite of somebody, but I think heterogeneity is a good thing.”

Showtime made one new series announcement on the unscripted side: 10-episode, half-hour, one-on-one interview series “Laughing Stock,” exec produced by Steve Carell and David Steinberg with Vance DeGeneres and Charlie Hartsock of Carousel Prods.

Steinberg will host interviews with A-list comic personalities including Tina Fey, Jerry Seinfeld, Chris Rock, Larry David, Ellen DeGeneres, Judd Apatow, Jane Lynch, Sarah Silverman, Mel Brooks, Carl Reiner, Don Rickles and Carell.

After “Homeland,” which co-stars Damian Lewis and Mandy Patinkin, the next high-profile launch for Showtime will be black dramedy “House of Lies” with Don Cheadle and Kristen Bell, which has been given a Jan. 8 premiere date between the season bows of “Shameless” and “Californication.”

I like shows that have some scope, that feel like they have some bigness to them and that have relevance to the world we live in today,” Nevins said.

Series will remain the bread and butter for Showtime, added Nevins, who has no desire to compete with HBO or others on the movies and miniseries front.

For us, it’s really about renewable resources,” Nevins said. “You fall in love with something, you stay with that subscription all year in order to get back to that show.”

On that note, it was interesting to hear Nevins reflect upon the cancelation of “United States of Tara” after two seasons.

It was a big show for us,” Nevins said. “Toni Collette won an Emmy, and it got real acclaim. You’ve got to make decisions about where you put your resources. We felt that show had accomplished what it was going to accomplish. … It was time to invest in and plant some new seeds.”

On the other hand, “Weeds” is in its seventh season, but Nevins is optimistic about its renewal.

I think that show’s got real life in it,” he said. “Jenji (Kohan) has done a really good job of evolving her show and making it move.”

Two other projects remain alive at Showtime but without scheduled airdates: comicbook adaptation “100 Bullets” — “a very important piece for us,” Nevins said — and Oliver Stone’s 10-part “Secret History of America.” Nevins said the doc was about halfway done but was interrupted by Stone’s feature film schedule.

It’s a labor of love for him,” Nevins said, “but movies are his day job and when you get movie financing, you take it and shoot it.”