Sources tell THR that HBO is searching for a new director (or directors) for a likely second season of the Emmy-winning miniseries 'Big Little Lies.'

HBO's Big Little Lies. Netflix's 13 Reasons Why. HBO's The Night Of. AMC's The Night Manager.

Each of the above dramas were designed (and promoted) as limited series — small-episode-order shows with a beginning, middle and conclusive end that wrapped the story. And yet all four of them are either officially coming back or in talks to return for second seasons.

In the case of Reese Witherspoon and Nicole Kidman's Emmy-winning Big Little Lies, sources tell The Hollywood Reporter that all that stands in the way of an official season two renewal is finding a new director. Sources note that Emmy-winning helmer Jean-Marc Vallée is not likely to return for a second season. HBO declined comment.

Meanwhile, HBO and star John Turturro are both optimistic about season two of The Night Of as a formal idea would likely lead to an official second season. The same is true for AMC's Night Manager, with producers in early development for a second season of the Emmy-nominated BBC drama starring Hugh Laurie and Tom Hiddleston. And then there's Netflix's 13 Reasons Why, with the controversial suicide-themed young-adult drama already in production on season two.

THR polled 17 network toppers (as part of our semi-annual executive survey) to find out when it makes sense to turn a limited series into an ongoing franchise. Their answers shed light on the creative and business process of going beyond the book (in the case of 13 Reasons Why and Big Little Lies).

When does it make sense to turn a limited series into an ongoing franchise?

Cindy Holland (Netflix): When it's good, people watch it and there is both an ongoing story to tell and the creators want to continue.

Casey Bloys (HBO): Only when everyone involved feels in their bones a potential second season is a worthy successor to the first.

David Nevins (Showtime): When the first one gets good viewership, and there’s an idea for the second season that actually has a strong relation back to the first.

Joel Stillerman (Hulu): When the talent availability and the audience response make it worthwhile.

Chris Albrecht (Starz): It's a function of, "Is it worth the investment?," and the two criteria would be, "What's the value of it to the network?" and "How good can it be?" If it's not going to be at least as good as the first time [there's no point], and sometimes you can only do it by trying. It's one of the reasons we did The Girlfriend Experience so different [in season two] — so that the comparisons are not fair and you have completely different versions. I hope the discussion is, "Here's what a male filmmaker did, and here's what a female filmmaker did."

Channing Dungey (ABC): So much of it for me rests on the storyteller's vision. Is there a new direction as compelling as the original? If so, then I'm pretty much in.

Jennifer Salke (NBC): When you see a huge appetite for the content and a thirst for more.

Mark Pedowitz (The CW): If you truly believe that there's a second season in it and you're not doing it just for commercial benefit.

Sarah Barnett (BBC America): When it has a sincere and genius creative energy behind it — like Jane Campion's Top of the Lake.

Courteney Monroe (National Geographic): It’s easier to expand an existing brand than to create one in the first place. When we see a spark of success, we always consider how we can nurture it into something bigger. With Genius, we knew we wanted an anthology series. Einstein worked. Picasso is up next. And you’d be safe to assume we are thinking about who else’s life and story will stand up to the Genius treatment next.

Keith Cox (Paramount Network, TV Land): If the story is still worth telling.

Rich Ross (Discovery): A limited series becomes long-running when the audience loves chapter one and wants that kind of treatment for another like story.

Debra Lee (BET): When it's a hit like The New Edition Story.

Kent Alterman (Comedy Central): I’m not sure, but I suspect it could have something to do with popularity.

Chris Linn (TruTV): When there is audience demand and more stories to tell.

Jennifer Caserta (IFC): When the creative dictates there is an ongoing series in the idea.

Bill Abbott (Hallmark): Consistent ratings over a few weeks.