An interest check is a resume from the GM. It's a chance for them to show you how serious they are about their pitch, how they write, and how they plan on running things. 

A check should have a catchy title to get me to click on it. Once I'm looking at the check itself, it should contain all of the relevant information I need. I want to know who the player characters are, what the setting is, just enough about the plot to get started, and for any special mechanics to be explained in great detail. When I read an interest check, I want to know the GM has a vision for their project and has put some thought into how things will work. If a check does all of that and sounds like something I'd join, I join it. The temptation is to post one of my own ICs, but instead I'll use [url=https://www.roleplayerguild.com/topics/187467-the-storm-cycle-reclamation/ooc]this one[/url]. Notice how we're given a little sample of how the GM writes and then they dive immediately into the player characters, followed by all the information required to make them believable. It's also formatted well.

I will join an RP like that a hundred times over something poorly formatted. Especially if the GM has little to no information about the setting we're playing in. I shouldn't have to ask any questions about the most basic elements of the setting. even in a fan RP I'd like the GM to point out any places where they choose to not conform to canon or any special additions they made. I won't even click on a check titled "______, Anyone?" or something equally on the nose. It shows a lack of creativity and such checks are often as basic as white bread.