Thrillers demand an edgy pace that seldom lets up. This sets them apart from crime novels that might have only fleeting moments of suspense and action. Like a thriller movie, the novel must keep the reader on the edge of their seat for most of the story.
Things that I've found helpful.
1. An appropriate writer's group is great for bouncing off ideas. By appropriate, I mean at least some of them should be writing in a similar genre and be at the same or an advanced stage to you.
2. Research is vital, you must get your facts straight, even in fiction. You cannot expect your readers to suspend disbelief if there are glaring mistakes in your story.
3.They say its better to plan your thriller before you begin, but this is a personal choice I don't always adopt. It is helpful I find to write at least a brief synopsis first. Preferably, a six page synopsis of your story, you may uncover possibilities for the plot as you go and you can bet you will change some of it. Thrillers often have complicated plots and it's too easy to paint yourself into a corner if you write by the seat of your pants, and believe me I've done it.
4. Beware of cliches - refer to the above. Cliched characters also.
5. Finished that final draft? Publishers today demand that your work is well edited. It's time for a rigorous editing, down to each line on each page of the manuscript. Search for spelling mistakes that spell check won't pick up, passive sentences and weak sentence construction, sentence variety and paragraph length. Chapter length - thrillers generally have short paragraphs and short chapters to increase pace. Check each scene for plot, pace, setting, point of view, dialogue, action, description and narration. Look for strong conflict and character interaction that builds a great story. Check for that favourite word or two you use without noticing, (we all have them) that pops up too often. That done, check formatting and the presentation of your manuscript before submitting it to a publisher.
6. An independent opinion is extremely helpful at some point, but be careful where you get it. Preferably from someone in the business, well worth the outlay if you can afford it. I had a friend who gave up writing a novel because she asked the wrong person's advice. Best are those who read the type of books you are writing. If you are writing a romantic suspense, don't seek the opinion of someone who reads only literary fiction. Not to say that romantic suspense novels aren't well written. It's a demanding genre to write, to find the right balance between romance and suspence. But feedback may not be helpful from someone who doesn't understand the demands of the genre. Bad criticism can be damaging to a fledgling writer's confidence and even steer them down the wrong path. If you don't agree with the feedback from a suitable source, seek another opinion. If the same criticism arises more than once, it might be wise to take heed of it.