Stay at one position,add custcene but don't move custcene just continiou pressing 'space' and on the end enter the text and press enter. When you press enter,don't press anything else that will move CJ,just get into meni and do it again and enter different text,or what're you want.

-This was simple and hard to do. I saw many topics in GTAF about this,so here's finally the tutorial.
Example mission download here.