A difficult scene to pull off, in any incarnation. Their decision has to make sense, and not be some decision forced onto them by a screenwriter. Granted, the scene from the cartoon can't be used in this context, since they haven't spent nearly enough time with Aang for their reasoning there to make sense.
I won't say mine is perfect, but I think it works in the context of my own draft.
The Legend of Aang: "We have to do something..."
INT. KATARA’S HOVEL – DUSK
KATARA stares up blankly at the ceiling from a cot, before hearing some shuffling below her. She looks to see it before facing back up again.
I’m sorry about what happened-
I just don’t understand what made you so against him. He did nothing to make you think of him as anything less than a friend, and we just let him get captured.
KATARA covers her eyes with her hands.
I know it’s stupid. But… What if he is the Avatar? He’s our last beacon for hope, and we both know what they’ll do to him.
There is silence.
We have to do something.
I couldn’t agree more.
KATARA widens her eyes and looks over the edge of the cot, to see SOKKA packing a small back-pack.
KATARA breaks out a near-shocked grin. SOKKA looks up at her.
Let me make one thing perfectly clear… I’m only doing this because I don’t want to go down in history as the one who gave up the Avatar.
KATARA chuckles and steps down from the cot.
And, if he’s not, you know… He is just a kid, after all. No right to what they’ll do to him.
KATARA grabs his hand before he can tie up the pack. He meets her eye-line.
SOKKA grins slightly and returns to tying up the pack.
So… You plan to become a better paddler spontaneously or do you have some other idea?
SOKKA looks up to face her, a small mischievous gleam in his eye.
EXT. SOUTHERN WATER TRIBE – NIGHT
APPA thrashes back and forth, as SOKKA attempts to console him by petting him. KATARA stands dumbfounded in the background.
Do you even know how to fly this thing?
I have an idea. I think.
SOKKA looks at her, as she still looks at APPA.
Come on! Let’s use nighttime to our advantage.
KATARA walks up to him.
Just where do you think you’re going?
They both stop cold, looking to KANNA as she approaches them.
Gran-Gran, please. Let me explain-
KANNA holds up a sheathed dagger.
You forgot this.
SOKKA looks over to KATARA, as KANNA breaks a small grin. He walks over and takes it from her.
If he is the Avatar, then this journey will be far more than simply finding a ship. You do not have to do this.
We can’t stay here. I want to help him.
Then I will not stop you.
KATARA and SOKKA walk over and give KANNA a hug.
I’ve always known you two were destined for greatness. Promise me you’ll be safe.
We will be.