Faster than light travel was not supportable, but a transitory state of a black hole could dilate time.

Thorne:

Let me just say as we start that I was doing a lot more than simply justifying the science in the film. The story was built from the ground up on the science to a very great extent, through brainstorming sessions I had with the Nolan brothers. There weren’t a great many times where I had to go in and explain things after the fact.

Scientific American interviewer:

Well, correct me if I’m wrong, but it seems to me that not all of the science is treated equally in the film, with the science behind its visual components being favored. Take Gargantua, the supermassive black hole the astronauts visit in the film. It’s a thing of beauty, not only aesthetically but also quantitatively, because as you’ve shown in the book, it looks like the real thing. That happened through a painstaking back-and-forth process between you and the filmmakers. But in your book you also mention that Christopher Nolan came to you with a “non-negotiable” and rather far-fetched idea for the astronauts to visit a planet orbiting Gargantua where relativistic effects make an hour there equate to seven years back on Earth.

Thorne:

You know, Chris also considered traveling through space faster than the speed of light as “non negotiable” back then, and that’s something that was changed and is not in the final film. He used that phrase in our brainstorms, but in the end after in-depth discussions he came around. We’d always find some way to make things work together, though in this one instance of faster-than-light travel I gave him a series of reasons why we were quite certain the laws of physics prevented it. We went back and forth for several hours on and off over two weeks about it, until he reached the point where he appreciated intuitively that the problems I was pointing out were insurmountable. Then he simply abandoned the idea of faster-than-light travel and moved in another direction.

This business of the enormous time differential between one of the planets orbiting very close to Gargantua and the flow of time back on Earth – the problem seemed to be that no planet could endure the resulting gravitational forces. This was something that even I thought was impossible, intuitively, until I went and slept on it and did a few hours of calculations. I came to the conclusion that in fact it is possible. The black hole needs to be spinning very fast, but is possible for the spin to be fast enough for a planet in the necessarily close, stable, circular orbit to not be ripped apart. I can’t fault anyone for saying, “Hey, that’s not possible,” without having first having the benefit of my book! Unless it’s someone who is very deep into general relativity and who I would’ve expected to go do the calculations!

For more scientific details on Gargantua and other phenomena depicted in Interstellar, see the source article here. (This link is also on the next page, so it may be more coherent to read the next page excerpt before traveling to this link.)

Click next page to see the discussion of Gargantua’s accretion disk (from the above-referenced article link), followed by a somewhat lengthy video of three scholars taking a look at whether this accretion disk could really exist. (Of course, they did not have the benefit of Thorne’s book at the time the video was made.)