The current project I am working on (a thriller that I’m adapting from my own screenplay) is really dark. I mean, dark with a capital D-A-R-K. Since it involves a series of murders done in various gruesome ways (mirroring that which has been done in cinema and TV, as well as real life), the mood can be quite grim.
Alfred Hitchcock, Master of Suspense, knew this very well, which is why he would inject light humor to release the tension in his audiences - then ratchet the terror, the suspense, and the tension to even greater heights. See Psycho (1960), Shadow of a Doubt (1943), even Rear Window (1954), for examples of this technique.
What helped create those moments of tension was the fact that the characters were perfectly normal - on the outside. Their inner realities didn’t show themselves until too late. Norman Bates could walk around, unremarked on, much like Ted Bundy or Ed Gein.
So, in order to balance out the darkness for the audience, humor must be woven in to break the tension, give a breather, and to allow that fictional world to be grounded deeply into reality.
The amount of research I initially did, when writing the screenplay, took me to the darkest parts of humanity. Many of the crimes I studied took place more than a century ago, some even hundreds of years ago. It is somewhat disconcerting and not surprising to learn that humans can be absolutely brutal to one another, no matter what century.
In any case, the research I did helped - the story spooked both my mother and my brother.
Which is actually a satisfactory response to a dark thriller.