Okay, I'll tell you.
Roleplaying is as simple as writing a story, just with other people with you through the adventure. You take turns in your posts and eventually you all start working together for a goal, with the exception of the Villians trying to stop you of course.
Controlling your character through text is as easy as writing:
"Bob looked at his watch and noticed that he was late for work.
Damm, he thought, Mr. Hutchenson isn't going to like me showing up to work late again."
It's as easy as that really, all you have to do is write what your character(s) do in your turn, adding detail of course.
Interacting with another Roleplayers character is easy, I'll show you through an example.
1.Henry unsheathed his sword and held it tightly in his right hand as he and his brother, Collin, prepared to enter the sinister castle of the Witch, Tsmarnia.
"I'll go in first." Said Henry. Henry knew that Collin was not as confident in his fighting abilities as Himself.
With his left hand Henry pushed open the old wooden door that was the entrance to the Castle, then he entered.
2. Collin watched his brother unsheathe his sword.
"I'll go in first." His brother, Henry, told him. Collin nodded and unsheathed his own sword, thankful that he didn't have to enter the castle of the Witch, Tsmarnia.
Collin watched Henry enter the castle, then he followed cautiously.
It's as simple as that, just read what other people have posted, then act accordingly