Casual is typically for those that like to write a paragraph or two. Or in short would rather not see one liners all of the time (even though every now and then would be ok.)   GM can choose his/her own standards. Like you can ask for at least a paragraph if that's what you wanted. 

Basically, most here like character development and some amount of depth. 

Most the time, you already post to casual standards. But for times when you can't figure out more than 2 words to post here is a few things you can think about: (which happens to everyone at some point, so don't feel bad when you can't think of anything.)

What is your character doing and/or seeing? What does your character think of the current situation? Basically get into your character's head. What does he/she see, smell or feel? Describe anything right down to the scenery. A character doesn't have to have dialogue in every post. There are plenty of ways to flush out a post. Most the time...it kind of depends on the circumstances story wise as well as how your character is developed at the time though. 

You as the GM can decide where the story goes. I've seen GMs (like Creator) with a full plan on where they want things to end up, but take suggestions on side plots and what not.

Hope this helped lol.