[duggable]

This worked on our (then) 5 year old, who was shy on the soccer field but kicked it up a notch when we told her to be a dinosaur. Of course, this also resulted in her running around with t-rex arms and baring her teeth, so I guess you have to be careful which alter ego you adopt.

[armandososa]

Your story reminded me of this comic http://hyperboleandahalf.blogspot.com/2013/10/menace.html which I like to read every once in a while.

Edit: And of course you already had two suggestions for this. I should refresh the page before commenting.

[MagnumPIG]

Goes to show how great that comic was.

[woliveirajr]

Yes, and indeed it works:

http://hyperboleandahalf.blogspot.com/2013/10/menace.html

[grimjack00]

At least she doesn't need the costume: https://hyperboleandahalf.blogspot.com/2013/10/menace.html