LOL yeah, people assuming the whole character on just a few clips? (not just here, in many trailer reviews)
Just cause in any trailer you want to tell a quick story narrative, all the Billy scenes they chose are him being intense for dramatic set up purposes, which then shift to when he's SHAZAM = ...is when all the fun begins!
At abt 1:30 you see a bit of (immature) Billy joking around too.
And in that you see Zach Levi actually surprisingly reads incredibly well as that same kid, in an adult form.