When your characters take over...

A question recently popped up on one of the children's writing groups to which I belong...
"I really don't know what my character's personality should be like. Please help?"
As is often the case with writers - we're a friendly, supportive bunch, see - suggestions flowed in thick and fast and included:

making your characters take the Myers Brigg test,
talking to your characters to find out more about them,
getting into your characters' head (I can't help wondering if one needs a ladder and a crowbar for such an exercise...),
studying psychology (a whole degree seems a bit extreme...) and,
writing about your character in the first person and getting them to answer questions like who am I? (which in my case might leave you in even more of a muddle).

Frankly, I find I tend to have the opposite problem. I have a really hard time getting my characters to stay out of my head, to stop them from taking over and demanding what they want. One character in particular has taken on a life of it's own... And is, I feel, entirely out of control. Now any advice on this, would be most welcome.

Ba-kaaaak! Ya talking 'bout me? Huh?
Who me? Talking about you?
Yeah!
I wouldn't dare! Honest.
Hmmm. Let's just get one thing straight for you and everyone else out there. I am NOT a figment of your imagination. I am real. I come from Novapulse. I'm a secret agent observing the human condition and my granny is a werechicken. Best you be afraid, very afraid!
B-but...
But what? You got a problem?
No, no, of course not, no problem at all.
Good. Glad we're clear on that.

Whew! See what I mean about characters being out of control?

I heard that!

Damn!

Tell me, does this sort of thing ever happen to you?

(This wire and beadwork chicken (and other similar creations) is made by local craftspeople and is sold at the roadside and local craftmarkets.)