Ondorus nods, and coughs into his fist. "...Prose is self-explanatory, and used in some form on most roleplay platforms. I think it's safe to assume you already know how to use it - it's the same style of writing used in novels."

Of course, he's not limited to expressing only physical actions in prose. Part of the fun of RP is knowing what's going on in a character's head, so if he imagines a cactus - like so - he can do that for the benefit of the reader, even if Frey can't reply to that.