I usually conceal it in some convenient location (such as up my sleeve) shortly before I know that I'll need to produce it. At the appropriate time I say something like "Oh? Do you need a pen?" and then while employing a bit of misdirection with my free hand I shout something like "PRESTO!" while I quickly retrieve the hidden pen with my other hand and have it seemingly magically appear before the astonished individual's eyes.
Actually the tricky part is usually making sure I don't poke the rabbit in the eye while concealing or retrieving the pen.
Goose quill pen? Fountain pen? Ball point pen? Retractable point? Write underwater? Write in zero gravity? Will you use ink already available? What kind?
We really do need some more from you, or there is nothing for us to do but joke around.