In The Stranger, what new knowledge do we gain about Mersault's character based on his interactions and relationships with Salamano and Raymond?
In The Stranger, Mersault's interactions with Salamano and Raymond illustrate Mersault's social indifference and refusal to question the meaning and motivation of other people.
Salamano is an old man who lives in Mersault's building. Salamano regularly beats his dog. Mersault greets Salamano and asks him what the dog had done, but he never really confronts Salamano's violent behaviour towards the dog. Mersault just accepts it as the way things are.
Mersault agrees to write a letter to Raymond's girlfriend despite the fact that Raymond admitted to beating her. He even agrees to testify against her on Raymond's behalf. Overhearing Raymond beating her again, Marie asks Mersault to call the police. Mersault refuses, saying he doesn't like police.
When Salamano's dog goes missing, Mersault observes Salamano's sadness and actually thinks of his mother. This does indicate evidence of Mersault's emotional side, but it is also telling that his identification is with a man who is sad for the loss of a dog he repeatedly abused. Furthermore, he doesn't give it much more consideration than a passing thought.
Through the wall there came to me a little wheezing sound, and I guessed that he was weeping. For some reason, I don’t know what, I began thinking of Mother. But I had to get up early next day; so, as I wasn’t feeling hungry, I did without supper, and went straight to bed.
In The Stranger, Mersault's interactions with Raymond and Salamano reveal that Mersault doesn't care about the violence these men commit or about why they are so abusive. Mersault tries his best not to disrupt bland repetition in his life, and he ignores any emotional or social impulse that will cause disruption even if such an impulse is the right thing to do (i.e. feel something at his mother's death or to call the police on Raymond). If Mersault stops to question why he interacts with these people, he disrupts that familiar repetition. Mersault does not want to question anything in life because he thinks it will lead to anxiety.
Ironically, his involvement with these two characters does disrupt the repetition. Salamano's dog goes missing; that daily abuse is gone. And most significantly, his "friendship" with Raymond leads to the shooting on the beach which disrupts Mersault's life completely.
Mersault believed life was meaningless, but rather than accept that possibility that life was absurd and try to make the best of it, he retreated to total indifference. The fact that Mersault even associated with Raymond reflects this thoughtless indifference and his (Mersault's) failure to summon the courage to live a life of purpose (even if it was in spite of his belief that life is meaningless).