Before I go any further I want to reinterate what others have said that if everyone is having fun, then there is no problem.
If you want to try and broaden the characters, here's something that may or may not help... Instead of trying to invent a character from scratch and think of everything he would do, try picking someone you know, either in person or an on screen personality (though probably not one who is super famous), and try playing that person.
Maybe you had a teacher in high school that was cool, mean, aloof etc. Or maybe one of your co-workers is always doing crazy stuff you would never do.
Or perhaps envision yourself as umm whoever that annoying brother of either the main character or the main character's wife/girlfriend (whatever, it's been a while, hopefully you remember who I'm talking about) in The Mummy / The Mummy Returns.
Or envision yourself as Teal'c (sic I'm sure) from Stargate-1 (asuming this is a D&D campaign).
I would advise againt portraying yourself as Indiana Jones or Luke Skywalker (whhhhyyyyyy... omg that would get annoying too) or someone that everyone knows immediately, as that stereotypes you in others' minds as well. Famous people outside your social circle is probably fine (like if someone acted like Martha Stewert in my campaign I doubt anyone would place it).
Using someone else as a template for your character allows some people to more easily envision what they would do and keep focus. For others it may just serve as a distraction, so your results may vary.
At least that's my 2cp.