Age certificates for Thrillers.

15:
Examples of 15 certificates Thrillers:
Taken, The Last Stand, Looper, Parker, End of Watch

All these films have crime in common, and they keep you on edge throughout the entire film.They are all action based and include a lot of swearing, violence and have sexual references.

18:
Examples of 18 certificates Thrillers:
Dredd, Killing them softly, Pulp Fiction, American Mary, The Departed

These examples are all classed under the 18 certificate because they are quite explicit in the contents of sexual references and acts, violence and gore, and drug abuse in detailed content.

The difference between these two genres is the difference between the physchological effects in the 15 section and the gore and violence in the 18 section.
The 15 films are most to do with the background of the story and figuring out the puzzle, with scenes of violence e.g. person-on-person fighting, group fights, car chases, car/building explosions etc. without the explicit content. And mainly references to sexual acts and prostitution and is very rarely seen in explicit content. Also drug use is only implied, and barely ever seen, if so it is referred to as being bad or it is the less lethal drugs.
Whereas in the 18 section,, there is still the physchological side, but it is made more explicit as it can include a lot more gore when violence is used therefore you will see the effects of weapons and detailed deaths e.g. a lot more 'blood' loss shown, missing body parts, snapping/breaking bones etc. Also sexual acts may be seen, and referred to in deatil and drug use can also been mentioned and seen.