...teach a man to fish...
Look in Interpreter 2 for "Show Text"
It will return a method, "command_###"
Now look in Interpreter 3-7 for that method to see what it does.
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  if $game_temp.message_text != nil
   # End
   return false
  end
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If there is already a message waiting to display, wait until that one's done (don't worry about this)
# Set message end waiting flag and callback
@message_waiting = true
$game_temp.message_proc = Proc.new { @message_waiting = false }
These are for the interpreter, so it knows a message is displaying, and can continue when the message is canceled. (also, no concern)
# Set message text on first line
$game_temp.message_text = @list[@index].parameters[0] + "\n"
This is the part you care about. @list[@index].parameters[0] is the string of text from the event command. It just assigns that text to
$game_temp.message_text.
So, if you just set:
$game_temp.message_text = "My New Text"
That should do the trick
Remember, if the statement is too long to fit on one line, you'll need to use \ to escape the carriage return.
$game_temp.message_text =\
"My New Text"
If you have more than one line of text for the message box, you'll need to use line-feeds...
$game_temp.message_text =\
"Here is some text. With a whole\n\
bunch of other text to make it a\n\
really long string. Add some more\n\
text to make it 4 lines."