Thrillers are stories that keep you on the edge of your seat, with suspenseful tension, but how does one accomplish this?
One could write hundreds--if not thousands--of pages on this topic, but let me use a metaphor to show you.
The tension in your story is a lot like fishing. When you snag that big fish, you don't just reel it in with all the tension and strength you can muster. Your line will break and the fish will be gone. The best way is to keep the tension on at times, while other times you let up, allowing the fish/reader to take a breather.
The same goes with your thriller. Pull the tension too tight and you'll lose the reader, wearing them out. Keep the tension too loose and your reader will go do something else.
Read up on your favorite thriller authors and see how they accomplish this feat on tension.