I love, love, love mystery suspense novels, thrillers and domestic suspense. I binged that genre right around the time 'The Girl on the Train' by Paula Hawkins and 'Reconstructing Amelia' by Kimberly McCreight came out, and haven't looked back since. The former became a movie, starring Emily Blunt, also really good; and the latter was optioned pretty much right away. Rumor had it that Nicole Kidman was attached to the project which would be released as an HBO limited series. The 'Reconstructing Amelia' project still hasn't come to fruition, which is too bad because the story holds up really well.
Go to Amazon and check them out. In my opinion, both books are pretty good examples of how this genre unfolds well, and both employed devices that I think can either make, or break a good mystery-suspense novel.
Device #1—The "Perfect" Husband/Boyfriend/Fiance. From page 1, we pretty much know that the perfection is a mask for something. Maybe he's the killer, maybe not. But if he's the killer, you'd better redirect us a few times and make us second-guess what we've decided from the beginning. And if he's not the killer, he still better not really be perfect because a perfect character is one that's really, really difficult to like or care about. Not to mention boring.
Device #2—Substance Use. 'The Girl on the Train' did this really well. The problem drinking of the main character was what made her an unreliable narrator. But her drinking cost her something other than the ability to remember key events clearly. In other words, the author didn't trivialize the drinking just to create an interesting plot point. It's always mildly disappointing when a character has a substance use issue that disappears once the mystery is solved, or gets a little too tidily dealt with once it's no longer useful for the plot. That's not just unbelievable, it's irresponsible.
Device #3—Tragic Secret. These are sometimes just tedious, to be honest. Sometimes it feels like a pile-on. Someone who has something incredibly tragic in their past then finds themselves in the middle of a murder mystery ... it can feel like (no pun intended) overkill if done poorly, and make a character read not as sympathetic, but just ... pathetic.
Device #4—Intrusive and Overly Concerned Bestie. It's always a good idea in general for at least one character voice aloud what the reader is likely thinking. You know, like the guy in the horror film who says to his stupid friends, 'hell nah, we ain't goin' in that dark, creepy house!' Sometimes the best friend who shows up periodically is that person. That's cool. Just don't have them show up excessively, or call excessively, or be excessively wise. Because then it's clear what narrative role they're playing, and they stop feeling like a real person.
Device #5—Hot-Mess Hero/Heroine. This is a corollary to the unreliable narrator thing, in some ways. But for once I'd like to see a strong, capable person in control of their life who nevertheless has something strange, or crappy and unsettling happen to them and from the outset, try to take control of their situation. I get that the hand-wringing thing is part of the suspense sometimes, but too messy and I don't know about you, but I lose patience with them.
Finally, can I say this? I'm working on a book in this genre now and I can definitely say that mastering these devices? Much easier said than done.