This is one heck of a difficult film to rate. Co-writer David Birke also co-wrote another serial killer biopic/psychological portrait, Dahmer (2002), and both films suffer from many of the same flaws. Gacy may have even more problems. There are countless things that could have been done better.
Yet in combination with co-writer and director Clive Saunders, Gacy manages to retain your interest, and excels at the prime directive of serial killer flicks--it makes the viewer feel profoundly uncomfortable. If judged solely on that aspect, the film would deserve a 10 out of 10. Of course, not everyone wants that kind of emotional experience with a film, but it seems to me that if a serial killer flick doesn't make you uncomfortable, something went wrong. The subject isn't exactly puppy dogs and pixie sticks, unless we're talking about barbecuing puppies and using the pixie sticks for spice.
Let's get out of the way that the film isn't precisely, historically accurate, and it's far more historically incomplete. I don't consider that a flaw. Saunders makes it more than clear a couple times that he's used facts about Gacy's life as inspiration. This is not a documentary, but a fictionalization--specifically it's "historical fiction". Gacy had a relatively complicated life, and understanding his crimes "realistically" involves looking at a huge time span of complex events. There's no way it could be done in 90 minutes, or even 180 minutes.
However, the events that Birke and Saunders choose to show too often seem random, and there's too much exposition missing. We get one scene of Gacy-as-a-boy with his dad, whom we see being mildly abusive. This isn't sufficient to establish anything significant about Gacy's youth. There either should have been more material like this, or it should have been dropped altogether and simply mentioned at some point, perhaps during a bit of self-reflective dialogue (which we get later anyway).
Next we jump to a screen full of text telling us that Gacy was convicted of sodomizing a boy and spent 18 months in prison. Then we jump again, and suddenly we see Gacy living with a woman about his age, two younger girls and an older woman. We can figure out that this is his wife (it was actually his second wife) and mother, and we assume it's his kids (they weren't, they were stepdaughters). Eventually we're told their relationships (except my parenthetical facts), but it doesn't help that it is initially presented as something of a mystery.
There's a general lack of exposition as exemplified above that makes the film play more surrealistically if you're not familiar with Gacy's story. Sometimes this works--the inserts of Gacy eating chicken and dressed up as an alternate world Colonel Sanders (Gacy's first wife's family owned a number of Kentucky Fried Chicken restaurants in Iowa) are particularly striking, even if the viewer can't quite figure out why they're present. But just as often the lack of exposition is more of a problem, as with the two hippie-looking guys who are staking out Gacy near the end of the film. It's never quite clear who they are, why they're around, or why in some cases they appear to have lawn chairs set up within about 30 feet of Gacy's front door.
There are a lot of interesting facts about Gacy that are hinted at but not shown very well. For example, he was actually well liked by a number of people and he was very involved with community groups such as the Jaycees at one point. His fascination with clowns was also much more bizarre than is shown in the film. He had unusual makeup that friends recommended he change because it had potential to scare children, and he was an amateur artist who painted weird but wonderful clown/skeleton canvases (well, I like it at least, but I have a taste for outsider art, including psychotic stuff). In conjunction with the clown fascination, Saunders employs subtle carnival music in the score at one point. This worked well, but would have been better if more regular and prominent.
What Saunders focuses on instead are those elements that provide that uncomfortableness I was talking about earlier. Gacy had a crawl space beneath his house that served as a dumping ground for bodies and that produced an infamous stench. Saunders dwells on the crawl space, appropriately. He also fills it with cockroaches, maggots and other insects. Gacy comes across as consistently pathetic, almost sad, as does most of the rest of the cast, surprisingly enough, including Gacy's family and most of his victims. It's difficult when watching the film to believe that some of the victims would make themselves as available as they did, especially over time, but this is based on truth. A lot of small, subtle "beats" add to the pathetic feeling, including the driving shots through the dirty windshield, and a lot of white trash characters who look unkempt, who drive wrecks, and who work in dilapidated environments. Even though I ended up wishing there was more of the carnival music, I also loved the melancholy score that is prominent about two-thirds of the way through the film.
While the film might not provide a lot of psychological insight into Gacy, if such would be possible--he truly comes across as very rational and completely insane at the same time, and it might have benefited from a more linear, in-depth look at some of the victims, the film still succeeds by delivering a deeply disturbing atmosphere.