I was a girl for six weeks some 2-3 years ago. I was cast as Old Man in Prelude to a Kiss.
I played an old man from New York City who wanders into a New Jersey wedding and kisses the bride. At that moment their souls swapped. For most of the rest of the play, I was a girl named Rita trapped inside an old man's body (no, I'm not old, it was all a theatre illusion).
Of course, I had to be convincing. It involved much more than pretending I was female in an online conversation. I had to study the manners of the girl who played Rita: I had to notice how she walks and how she talks (oh, yes, women modulate their voice quite differently from men). Of course, she studied me, since for most of the play she was a man in a girl's body.
It was quite an enlightening experience. As a method actor I tend to act like my characters even outside of the theatre. At that time I used to go to a coffee shop daily. I became very perky for those six weeks (I am not saying all girls are perky, but Rita as portrayed in the play certainly was).
People who saw the play with me started calling me Rita even in the coffee shop, and commended me for being a very convincing girl. Indeed, several months later a total stranger stopped me on the street and said that even though he saw my manly appearance, he still thought I was a girl during that part of the performance I was Rita.
Naturally, I do not act like Rita anymore, but I sure miss her!