I've never had the headvoice experience, quite, where a character goes MEMEMEME. With the Master, for me, it's more that I've played him for about a year now, so playing anyone else means making an effort, heaven forbid, and he fits so well. I can use the way I think to write him, and it meshes. It comes easily.

I do have periods where other characters are 'stronger'—I'll really wish for a chance to play my metacrisis, for instance (I don't know why, but my metacrisis is the one I usually have brief I REALLY WANT TO DO SOMETHING WITH ASH periods. Probably because she's very different than the proper Time Lords). But the Master is a baseline. I rarely go HELL YES, I want to play you. It's more, oh, I'm RPing? *whips out the Master voices by reflex*

tl;dr!