its really all luck of the draw. when in detroit with a friend in 2003, we decided to head over to windsor for an afternoon. we were wished a good time by the canadian border patrol on the way there, but had a hell of a time returning to the united states. most likely, the u.s. border patrolman we encountered just liked giving people a hard time. while i didn't have my passport (called ahead and was assured i didn;t need one), he spent quite a bit of time scrutinizing my pennsylvania driver's lisence for authenticity (despite the very difficult to reproduce hologram on the entire front of the lisence and the magnetic strip on the back that if he just ran through a computer would tell him everything he could possibly want to know about me).
best to just treat it like a job interview. you will want to make a good impression, but don't say more than you have to.