There will still be lines - at least - for Finding Nemo, Indiana Jones, Peter Pan and Space Mountain.
On very busy days, it's also not unusual to find lines for Alice in Wonderland, Dumbo and the Matterhorn.
Most everything else does become a walk-on.
Splash Mountain becomes so deserted that its not uncommon to be able to ride twice in a row and Pirates is so empty that a request for a "private boat" might very well be honored.
So this doesn't apply to summer as much as it does to fall/winter/spring. In the summer, the mix of crowds is more out-of-town than local, and out-of-towners tend to stay late. Also, on Friday nights are the busiest - local APs often not blocked out, so locals come after work for the evening, as opposed to Saturdays when local APs are often blocked and Sundays when locals have school/work the next day.