Well, first of all, do it. Don't hesitate much, remain humble even should you make the transition with applause and cries of joy (as if this happens anywhere), and keep flexible to learning. That's really all you need. I'll tell you, when I first came here, I went directly into Advanced not because I thought I was a snapping writer or able to write very long, but because I knew I couldn't/wouldn't post every 2 days. I liked the idea of creating something larger over time.
What you really want to do is sink into the character. Spend a good amount of time in that Character Sheet until you're really interested in them and are curious about where they might go. For me, I wouldn't bring characters from other places into others, because I want to 'find', not create, the person in that reality. There's something to really getting sunk into those shoes. Remember that you are not the character. You're really a witness to them in this place and could mean they'd do things you wouldn't agree with, in any way, and maybe even things you know will likely lead to them doing something horrible or dying. You're just hear to write what you see. Now that can mean some internal monologues, but it's your job to put this down as eloquently and concise as you can. Do that and get back to the situation, the surrounding, the little gestures that add image to the story. Beyond length -- fuck length, this gives you substance. And that's coming from someone notorious for long posts.
I can be pretty short about dialogue. PM the other person and decide how y'all want to deal with it. I work pretty loosely with Free Faller, who I trust very much with my character in Apocalyptica. She's read my character's thoughts and speech, and sometimes she just glosses over his words (ex. Simon gave the usual speech, policy this, law that...), or simply had him respond as needed. Never something declaring his personal values or whatever else she might feel comfortable drawing from him. No matter what she does, or what I do with her character, we still send the post via PM prior to posting. This way the person can edit the speech if they feel the voice is lost. This allows all that necessary, beautiful dialogue, to happen in one person's post while keeping the spirit of each unique character.
There are some great GMs out there who I trust pretty well. Wonderful thinkers and flexible storytellers, I'd jump into any of their worlds. Find someone you feel that way about, maybe shoot them a PM, and don't try to un-sell yourself. Sure, tell them about your concern, but speak about your strengths or ambitions. Get some trust there and jump in. You will not get into Advanced unless you walk yourself in. If you don't meet the official guidelines, you'll meet that challenge. But you can't if you don't put yourself in the position to do it. So do it.