While searching for more information on that, I found these even-more-interesting tidbits at Out of the Past, many of them gleaned from commentaries on the DVD:
When Yuri and Lara first cross paths it's in a trolley. They don't see each other but when they touch accidentally the electric line above the trolley gives off a spark which symbolizes the emotional spark between those two characters.
David Lean's favorite color was yellow and he used yellow flowers, especially sunflowers to represent beauty and hope in the film. In the scene where Yuri and Lara part for the first time, sunflower petals fall mimicking tears.
[Director David Lean] really wanted to focus on her eyes but felt that Christie's soft pillowy lips took away from them. So throughout the film you see that shadows cover the lips or that the light specifically highlights the eyes versus the lips.
David Lean felt that it wasn't necessary to show violence that reaction shots were more important and added more to the drama.
The woman with the dead baby that gets pulled up onto the moving train by Yuri, the actress actually fell under the train and her legs were seriously injured. They used the actual shot of her being pulled under the train.
Author Pasternak's love affair with his mistress Olga is the basis of the love story of Yuri and Lara.
For the scene with the town of Yuriatan burning in the background, Lean had a crew burn 2 tons of rubber tires!