He sat down beside Christina, propping his feet up on the railing, feeling the warmth of the sun against his face, able to relax for the first time in days.
本句，the sun 指 the sunlight。against his face 意思是“照射在他脸上的眼光”。against 强调动作 “对着，迎着” 之意，而 on 强调“接触后的状态”。本句虽然可以换为 on 或 upon，但意思有上述差别。
以下两句，则不可更换：
Lilacs grew against the side wall.
丁香树倚着边墙生长。
《柯林斯英汉双解大词典》
She bounced the ball against the wall.
她对着墙打球。
《牛津词典》