Thrillers and mysteries are different, right? They must be because both categories have their own awards and their own conferences (ThrillerFest for thrillers and Bouchercon for mysteries). But I think the distinctions are often so blurry that they often serve little purpose other than providing a hook for the marketing efforts of publishers.
The thrillers I like best usually have some sort of whodunnit element and plenty of twists and turns. Likewise, the best mysteries usually have some of the forward momentum, action and immediacy of thrillers. But, that said, let's forget about those subtleties from here on out.
Imagine, if you will, a generic mystery/thriller protagonist named Sam Dekker, ex-Special Ops and recovering alcoholic turned PI. Let's consider what Sam would do in a situation if he found himself in a similarly generic mystery or thriller.
Scenario 1: Sam attends an AA meeting and meets Angela, a troubled young woman.
Mystery Version: After the meeting, Sam and Angela have a cup of coffee, talk about their shared problems with alcohol and commence a tender, tentative relationship. Angela happens to have a sister who has gone missing and Sam agrees to take the case. Then Sam sleeps with Angela.
Thriller Version: The faces of the attendees at Sam's AA meeting look strangely familiar. He realizes that he knows most of the people sitting in the circle of metal folding chairs from his days in Special Ops -- the meeting is a trap set by a bunch of old enemies. Sam pulls his Beretta 9 millimeter, just as everyone else in the room draws their guns. Carnage ensues. Sam is the last person standing, along with Angela, who seems to have been the only actual recovering alcoholic in attendance. Sam and Angela flee the scene before reinforcements can arrive. Then Sam sleeps with Angela.