If they're not playing their characters, then how come Ian Ziering did Steve Sanders' infamous tiger growl?

Keep in mind this was NOT during any of the dream sequences where he and the cast were in character, but instead where he was just playing the heightened version of himself. But, he did Steve's tiger growl. I'm not understanding here. On top of that, it seems like the whole cast was basically playing their characters, despite being referred to by their real names. For example, during a verbal altercation in the pilot episode, Jennie had mentioned how not very bright Tori is. Isn't that basically Donna Martin? It's like the cast was still in character despite this being a reality show.