Sorry if a lot of the below will be obvious, but I'm just winging this post, so it might contain a mixture of things that you already know and things that might help.

I found that when I play games that involve decision-making (such as Mass Effect, for instance) that there is a lot of myself in those characters and the choices they make. I think that even my FL chars have a lot of common with me (it might be a wrong approach, but heh, it works for me).

The most important thing is to act as your character would, so you need to know what he or she would do under certain circumstances. That's why yourself could be one of the easiest roles you can play. If you do that though, make sure not to use your actual name, origin, and appearance, that would be stretching it too far.

Note that the fun part of RP comes from being someone you aren't, so don't be afraid to work in some character traits that you're comfortable with (those can be borrowed from movies, books, games, etc.). The important thing is not to overdo it and not to import stuff that wouldn't make sense in Freelancer universe (playing, replaying, or watching the Freelancer campaign can help with that).

With that figured out, determine the basics of your character: what they do for a living and where they come from, that sort of thing. At this point you don't have to go into too much detail about your characters; they may as well get fleshed out along the way, as questions are asked (once you answer those questions, it's set in stone though). Being inspired by characters from other media is ok, as long as it's subtle enough. I've just figured out for myself that subtle comes down to not importing character names entirely. Traits and/or pictures are ok.

In a nutshell, here's what it comes down to, in my opinion:

1. Know the basics about your character and pretend that you're playing a role on stage (stay in character).
   
   
2. There's nothing wrong with learning more about your character as events unfold.
   
   
3. Add some character traits that you're comfortable with and can maintain.
   
   
4. Make sure that your character can or could exist in Freelancer universe. FL campaign is a good reference. Imports are possible, but should be subtle, so as not to ruin immersion.
   
   
5. Always stay in character (it helps with immersion; you're always on set, acting).
   
   
6. Grammar is important for immersion. Mistakes and typos happen to all of us, but proper capitalization and full stops alone can help a great deal.
   
   
7. Most importantly, HAVE FUN! Because that's why we're here for.

I hope this helps. As you play, you may want to develop your character further, list their goals, strengths, weaknesses, goals, etc. I don't think I ever went that far though, I mostly keep things in my head.