The concept is that Marissa is moving in with Alex and feels just completely overwhelmed. The new feelings, the scars from her relationship with Ryan and being independent for the first time make her feel scared and worried. She needs Alex to help her just let go and embrace her true feelings.
Also I changed how Alex and Marissa met.. cos there was enough of Ryan in the real OC.. ugh. So in mine Marissa is outside the Baitshop and Alex barges out mistaking her for someone else. When Marissa turns around Alex is slightly embarrassed and shy because well duh Marissa's hot, and she bolts. Marissa watches her go but that brief meeting changes her entire world.