I wonder if the romance genre doesn't much lend itself to tertiary characters with delusions of stardom. Something I say because I'm with Jeffe on this one. Tertiary characters? We don't need no stinking tertiary characters. Mainly because there's just not space. We've got internal conflict reflected in or exacerbated by the external conflict. We're maximizing hero and heroine page time. Or hero and hero. Or heroine and heroine. Or any combination thereof.
Secondary characters? Absolutely. Even the most committed of romantic partners need challenges and/or narrative outside of the primary pairing. Unless this is where we're talking 'tertiary.'
Anyway. I am 100% guilty of grooming my secondary characters to become the primary characters of their own novels. If I've ever had tertiary characters, they were zombie squirrels, and even then, I feel like those where more plot device than anything else. In Enemy Games, Silver City might be a tertiary character. I wanted the station to have personality - for it to feel like a familiar city with idiosyncrasies all its own, but there's no danger that I'm ever going to have a space station be the main character of a book. I don't think. Granted, as I type that I kinda want to - just to see if I could pull it off. Maybe I need to get out more.
What does everyone else think? Do romance writers have to spend enough page space on other issues that we don't have room for tertiary characters who want to break out of the background? I'm trying to think of anyone I've read with a background character chewing the scenery. If there is one, it'll come to me at 3AM. I won't get up to tell you about it. I know I haven't yet had a character try to take over a book. So far. If that ever does happen, I'll probably have to bargain with the character - behave and I'll get you your own book. Or novella. I look forward to having to deal with it if ever.