Shingeki No Kyojin Ova - Episode 1 -

This was the first time in the series a "mindless" Titan was shown to have the capacity for speech and complex emotion (reverence).

The discovery of the notebook and the evidence that Titans might be sentient changed the Scouts' approach, finally convincing Erwin to allow Titan capture missions. Watch Information Shingeki no Kyojin OVA - Episode 1

The episode follows two parallel timelines: the present search by the Scout Regiment and the past account written in a diary. This was the first time in the series

The episode introduced the name "Ymir" long before its significance was fully explained in the main series, hinting at a deeper history behind the Titans. Shingeki no Kyojin OVA - Episode 1