I don't know. I just...do. Usually I'm inspired by something. I see a cool picture, an interesting TV show or Movie. I hear a cool song on Spotify or YouTube and construct a character around that. I've got like 8.4 trillion characters at this point and a graveyard of dead factions behind me because of this...though mostly because I have ADHD and can't commit to one character.
Some of how I get into character is just reacting to things going on. Some characters I wait to be acknowledged (such as Crowley or any of my LSF characters) and then I respond. With SLRC or Starfliers, I just get into the mindset of Trek and come up with random science tech gibberish to spout off to confuse people. On the forums I usually just do comm. or log posts. Rarely will I do a story post anymore mostly due to time constraints because of work.
If I could give any one piece of advice, I guess I'd just say go with what you find cool. Listen to a cool song that you like and try to think of a character in a cutscene, movie, or TV show that you could envision them in with the song playing. Watch a TV Show or Movie. Read a book. Get inspired. Don't copy characters verbatim but use them as inspiration. That's the best advice I could give.
