The “secret” to the ability for Disney animators to achieve the groundbreaking realism of look and movement in animation was the tracing of live actors movements (rotoscoping). While this technique was well known someone put the live action footage together with the finished product.
These have been around for a while, but I just found them. As a big Disney fan these are so fascinating to me.
These were found at Buzzfeed here. That link also has video footage that’s worth a look.