you cant find a mental disorder about pretending to be other characters/people\role playing or in your words "I will still have whatever personality I'm "playing" at the time." because this isnt a mental disorder here in the USA. its considered a learned behavior, therapy technique, acting technique and many other terms. in other words here in america this is considered to be a normal part of life and taught as a therapy technique and with in many different careers like acting.

children learn to do this here in america the first time they play house or dress up or lets play pretend in early years, preshool, elementary. high schools also teach pretending to be each other and characters for their school plays/musicals.

most people know how to control when and how they pretend or role play based on who they are around, what is expected for a person to behave\act like in all kinds of situations. it can become a problem when someone uses this normal skill at the wrong times...example pretending to be their drunk relative in church, or pretend to be a comedian during a serious point at work.

that said this kind of learned behavior can be controlled just like when a smoker gives up smoking or a nail biting person decides biting their nails habit is too much for their life or an adult with a thumb sucking habit gives up thumb sucking.

notice when you are doing it. decide whether it is appropriate for you to do what you are doing at that moment. if its not and interfering with your life at that moment, stop yourself by saying something to your self like this isnt really how I am, its ok to be me and I can do this, then talk and behave like your self rather than the character\personality you have slipped into being.

example if I am at work and someone says something that makes me slip into comedian mode I think to myself this is not the time for this, focus on this present moment what do I need to do, to be myself and get on with my job? then I do that rather than the comedian role. over time I stopped slipping into comedian role at work. it took me about two weeks to control this. just like any other habit it takes time.