Like James Joyces' Ulysses, The Outsider seemed to be written like a thought process.
The style was disjointed with short snappy sentences. With quotes like "With the 2 o' clock bus...should get there before nightfall" it's like the reader has entered the characters head, with all his reassurances, worries and random thoughts.
The theme of existentialism runs throughout the book with ideologies like time being completely irrelevant to the story. The characters can exist and react in a sort of suspended reality where there is little to no past guilt and no thought of future consequences. The present time is also non-existent. "Mother died today. Or, maybe yesterday".
In terms of the future, Meursault seems to not even have an inkling of it when he is faced with old people. Rather than understanding that they are people and a majority of us will face old age at some point, he regards them with interest as almost being separate entities.
They were "noiseless" and "grouped around the keeper". He can't decide whether "they were greeting me and trying to say something". He exists in the present and therefore old age is something non-existent in his world. Also the fact that he hasn't seen his mother for years implies that he probably didn't witness her aging either.
Later on Meursault agrees to marry Marie just for the sake of it with little care for the actual ceremony and sacrifices of marriage in regards to just answering the simple question of "will you marry me?".
His consistently living in the present means the things that occur to him right then and there he will become consumed with. When a woman cries at his mothers funeral he feels like "she will never stop", this is probably just because Meursault has no comprehension of a few minutes time when she no doubt will. In a restaurant Meursault becomes randomly interested in a woman who he coins "the little robot". She has funny movements and he follows her out of interest until he can no longer see her again at which point she is forgotten about.
In existentialism the past is pure guilt. To live a peaceful existence you need to have no recollection of the past and no thought for the future, which in itself is dread. Meursault epitomizes this after he has killed the Arab. It isn't until he is being interrogated that it suddenly dawns on him that he took a life. Even then it comes across as nothing more than a fleeting thought, a sort of half interested realization of what he has done. If it wasn't for the rules and regulations of society forcing that emotion in him, it wouldn't have phased him. It also can be assumed that as he is shooting the Arab he is not thinking about the impact that would have in the future in his own life.
His detached emotions are summed up right at the beginning of the book when his mothers hearse casually reminds him of nothing more than the pen trays in his office at work. Things are very mundane to him and no object or item is more significant than the other. He also displays emotional detachment when it comes to Salamano abusing his dog and Raymond beating his girlfriend. Although it is noted at one point that when Salamano loses his dog and begins to cry, Meursault thinks of his mother. Though he himself makes no connection. He also comments at one point "one can't help feeling a little guilty I suppose", like emotions are things to be embarrassed about.
According to existentialism; guilt, fear and worry are all things that come from others. "Hell is other people" is a quote that epitomizes the existentialism view. Meursault has a few examples of his own although some of the time, the judgement from others is in his head. When the old people appear at his mothers wake he notes that "they came to sit in judgement of me". When he asks for time off of work he "had an idea that he looked annoyed and I said 'sorry sir'". When he reaches the old people's home he comments about the warden: "I had a feeling he was blaming me for something". Even after his mother's funeral he leaves the grounds to go for a walk and thinks to himself "what an agreeable walk I could have had if it hadn't been for mother". Although the premise is that he is still feeling grief and therefore can't enjoy himself, the phrasing and wording of it sounds accusatory, as if it was selfish of her to die at that point in time when he could have enjoyed a walk.
In the world of the outsider, there is no affection; merely getting "used" to each other. There is nothing remarkable in the world of Meursault. As Edmund Husserl, a phenomonolgist once remarked: when objects are paid attention to is when they become a problem. If you just breeze through life and pay no attention to the world around you, the past, the future, or even the present that much, you will be the perfect manifesto of existentialism.