Maybe is the title a bit confusing or at list showing a confusion about the "In Character" IC acronym.
/tell is used to talk between characters. IC is something else. It is about talking in the role of your character instead of chatting like ta player (this being OOC=Out Of Character)
In fact, it is often considered that the /tell communications are most of the time OOC. Because there is no explanation for your character to this possibility to talk and hear people talking to him from any great distance and even from the death realm.
Are characters capable of magic or telepathic communications? This is not specified in the settings.
Now, about private conversations (IC or OOC) in /tell, tuning the options can make it very easy.
Personaly I use [Tab] key to auto-reply tells, and [enter] to exit the chatbox.
[tab] is also very useful when typing the firstname of your correspondent when already in the chatbox. Just type the first letters and [tab] will try to auto-complete it. It will look for firstnames in your buddylist and the ones you heard talking in any chanel.