You set up the role of the character when you create it. If you step outside of that role, you are not roleplaying.
So characters never change during the course of a campaign? From a fictional standpoint, that sounds like an absence of character development, i.e. boring. From a rules standpoint, that's contrary to the way a PCs evolving role is baked into more than one edition of the game. For example, AD&D assumes a PC begins little better than a common peasant, grows into an important landowner/political figure, and finally into a demigod capable of going toe to toe with mythological figures. That's, umm, more than a single role.
Here's the best role-played PC I've ever had the pleasure of DM'ing for, from my old 2e campaign. Z. began as a street thief from a stereotypical "city of thieves". He was designed to be an assassin, a professional strangler and a bit of a coward (he's probably claim it was just prudence). By the end of the campaign he was a diplomat, a nation-builder, a husband to a neutral good princess, and something of a religious philosopher on a quest to find the true nature of the setting's 17 gods. A little like Diogenes, if Diogenes carried a garrote instead of a lamp.
This change happened slowly & organically over the course of several years of real time. Z. grew, just like a good character in a novel. Or a real person in real life. This is bad role-playing?
To reference some D&D nerd faves from other media, is Londo Molari from B5 a bad character? Are Bilbo and Frodo?
Roleplaying = playing a role.
The role is also defined during play, over the course of the campaign, by the choices a player makes. Also, tautology is no one's friend, with the possible exception of Gertrude Stein.
edit: just like in the endless 5 INT thread, I'm noticing the people with the strongest opinions on what "good role-playing" is aren't the one's providing any examples. Funny, that.