How to make only the parts of an object overlapping another one appear?

[Ibis]

let's say I have a dialogue box, I want text to scroll onto the dialogue box from below, but it should look like this:

and not this:


what's the easiest way to do this? presumably the dialogue box is going to be in front of the background layer but i want the background to overlap the parts of the text not inside the dialogue box

thx~