In last year's production of Macbeth, I came on in Act II, as Macduff, and in order to demonstrate that Macduff was a proper straight, regular bloke, I decided in rehearsals that he'd just come back from a beery evening with Lennox and would take a leak up against the castle wall, showing just what a manly, manly man he really was (What's that you say? Overcompensation?).
Anyway, at the dress rehearsal, I was distracted by trying to remember the lines in order and on making my entrance, went to the back wall and mimed unzipping the old bags and whomping out the python (you see, I can "do" straight man vernacular). Except that my mind was elsewhere and I entirely forgot to mime, slightly to the surprise of the people standing in the wings. Luckily I came to myself before I actually followed through.
...and it's odd because I was much too nervous to get my dick out when I did Equus, ten years before, when it was actually called for. Method acting clearly comes unbidden.