Stuck in the middle with me

I admit I've been spending lots of time thinking about the structure of my screenplay and I'm...well...stuck. I don't know if it's because I don't know the characters as well as I thought or if it's because I don't have enough story, or if it's something totally different. I know my beginning. Given the genre and the target audience (sci-fi/adventure/family) I know my ending.

But what about the middle?

I'm going to start looking at this from a different perspective: what does Rodney (the MC) need to learn in order to get to where he is in Act 1 to where he is at the end? What actions does he need to take, what does he need to succeed and fail in in order to get him to where he needs to be at the end?

Wish me cyber-luck!