If you would like to post your character's thoughts, you must differentiate between whether it is visible on their expression or not.
I've seen people use fake HTML tags like:
<thought> I think I'm going to go shoot that guy in a few minutes </thought>
or
[expression] Man! I'm so hungry. [/expression]
You don't have to use these in particular, but please make it clear whether your thought is written all over your face; or if it's truly part of your internal monologue.
Expressions will be considered to be IC knowledge, where thoughts will be OOC knowledge. If your PC has either at least one dot of subterfuge or a manipulation of 3+, you may post false emotional cues or body language for your character. Please do not include indeclinable OOC information in the body of your text. For example, do not refer to another character by their age or dark past unless it is posted in their description. This DOES fall under the heading of metagaming. If you're confused, see the section on that.