Insomnia shows one of the dangers of writing the thriller form. This is a very popular genre, but it is extremely narrow. Most writers of thrillers have great difficulty creating plot because there is usually just one suspect. The audience learns fairly soon who probably committed the crime.
In this film, casting compounds the problem of the single suspect. The audience knows you don’t cast Robin Williams as the bad guy unless he’s really the bad guy. The only surprise comes from how the opponent attacks the hero – which isn’t much – and how the hero catches him.
The writer tries to compensate for this basic deficiency by making the hero morally culpable. But the result is that both the main plot line and the hero’s moral decline are given short shrift.
Bottom line: if you are going to do a thriller, pay the dues and do the genre right. Hit the beats that make it a surprising plot. Without them, you have a small film that lasts way too long.