I can't review your site - I can't see it

Oh, this article looks interesting. It might solve my problem. I'll have a quick re.. NO I DON'T WANT TO SUBSCRIBE TO YOUR EMAIL UPDATES. Has anyone ever actually engaged with a website overlay, beyond searching for the tiny "close" button in annoyance? They are all over the place so there must be a good reason for them to exist. Maybe if we look closely at some of the steps required to make a successful one we'll start to understand.

Spoiler warning: we wont. Overlays are inherently terrible.

Making a successful overlay #1 - the timing

The overlay needs to appear before your victim leaves the site, so it needs to be quick. Never mind that "quick" roughly corresponds with the amount of time someone who is genuinely engaging with your site takes to orientate themselves and settle in to proper reading. In order to catch those people leaving quickly because they don't care anyway you're going to have to annoy your real visitors. Omelettes, eggs and all that.

Making a successful overlay #2 - make it intrusive

You're trying to catch the attention of your victim and what is more attention-grabbing than the entire screen being replaced by the action you want them to perform? Maybe it should flash too. Make sure they are under no illusions - your only interest in them is harvesting a review or an email address. Any content on the site is strictly secondary.

Making a successful overlay #3 - make it alien and hard to close

We don't use javascript alert popups these days because they are so easy to block and everyone has the location of the close button hardwired into their brain. However with an HTML overlay you can avoid this problem so your victim doesn't miss the information that is so crucial you have to interrupt their browsing. And it is, after all, all about you. So why run the risk they can easily dismiss your overlay? Why use established design patterns to put the close button somewhere obvious? Make sure it's carefully hidden and don't even think of adding a keyboard shortcut to kill it. That would be madness. The "escape" key has much better things to be doing.

Making a successful overlay #4 - make sure it is in the wrong place in your visitor's journey

You can't possibly wait until your victim has finished reading the page before asking their opinion or for them to sign up. They might leave at the end, having learned whatever they wanted, possibly even happy - and all without ever giving you their email address. The horror. Never mind that five seconds into a visit nobody has a useful comment beyond "the overlays annoy me" and that putting them right at the start of the visit means your overlay cannot possibly do the job it is supposed to do. Annoyance is valuable feedback so it should be gathered as early as possible.

Make sure you're considering these crucial four points next time you want to really annoy the poor souls who find your site!