I've done this a few times.
I'll give the guest a general set of guidelines on how to behave, when to back off, priorities and goals.. and let em at it with whatever personailty they choose.
Sometimes I provide the stats for the encounter, other times I just say, "I need a ___ ~CR __ is fine." and let em make up whatever they want, revise it a bit for relavence, and let em go for it.
If it's a real important encounter, I'll retain control, but a guest as a minor or random encounter spices things up quite a bit.