There's nothing wrong with starting from a base that you are at least a little comfortable with, then building the character from there. I don't think anyone starts out knowing exactly what character they are going to portray. My original character was a time-traveling psychic, it has since evolved to be a modern day witchdoctor. Though I do occasionally make references to time travel.
Originally Posted by MentalMark
Yes, the psychological manipulator is a bit played out since Derren Brown got big. However, there are still guys out there making it suit them. Paul Brook has a very psychological way of performing. Ben Cardall is the Sherlockian Mentalist. Colin Cloud is the Forensic Mind Reader. Huh - I just realized how many of the guys I look up to are British. So it's possible to define it further to suit you better.
When you do mentalism you are somewhat restricted to take either a scientific path, or a mystical path. I wouldn't say that's completely set in stone, but if you don't decide you're doing one or the other it tends to muddy the presentations. Actually, Derren Brown has just done a Penguin Live lecture and he talks about this. I recommend it. Regardless, you need to find a claim and study it enough to be able to hold a conversation with other people who have studied it. That means if you want to say you're reading palms, you darn well better be ready to talk shop with palmists. If you claim to be using psychological methods, you better be able to hold your own in a conversation about psychology.
So! How do you choose a character? First - think of something you think would be really cool to be able to do. An easy source of inspiration is comic book characters and literature. I'm heavily influenced by Neil Gaiman and the Urban Fantasy genre, as well as Abney Park's music and the steampunk scene in general. Throw in a healthy dash of Burning Man, and a sprinkling of Nikola Tesla and there you go - That's my character.
Sometimes it helps to find a single piece that is something your character would wear or carry or do. I can honestly say that the cornerstone of my character developed from a single routine. My Energy Transfer routine. That gave me the desire to emphasize connection, energy and the borders between science and mysticism. I started wearing simple, black clothing to keep the emphasis on the volunteers instead of me, but I also started wearing a black coat with bold red sections to stand out as an unusual character when not performing. On stage I usually end up removing it. From that routine I knew the flavor of the presentations for everything else I did. From there I built a show and truly became The Witchdoctor.
So think about what you want to do. Imagine yourself on stage in front of your audience, doing a great show. What's in that show? What's your idealized 'perfect' routine? What does it look like? What do you look like on stage? Think about those things, and then develop from there. And understand - it's probably going to take years to really develop the character until it fits you well.
The HP Lovecraft of Bizarre Magic