3: Stay in character.

Whether you're roleplaying as a fanon character, canon character, or an OC, it's important to be in character. Don't say you're roleplaying as a canon character, then make your character do stupid things they wouldn't do. 

Roleplayers like Arthur_Kirkland_ are exceptions. They don't add much differences to the canon personalities, so they're okay.

((If you don't like me using people as examples, please say so))

Most importantly, understand your character and act as though you are your character. A good way to get to understand your character more is to note down some things about their personalities and copy down their quotes.

For example, Noriaki Sugiyama/Sugiyama Noriaki, APH Britain's Japanese VA, does that for his character, and now he's the top second best person - behind Himaruya - who understands how Britain would act.

Basically, try your best to not stupidly roleplay out of character to the point of cringiness.