Jean-Paul Sartre probably communicated his ideas most through fables and tales. Although there may be a few students out there who have slogged their way through Being and Nothingness, most people's introduction to Sartre probably comes from No Exit, or another one of his literary works. This perhaps has more to do with people's attention span when confronted with philosophy than with Sartre's own efforts: since Sartre often was a very detailed (and indeed tedious) philosophical writer.
So for those who can't quite parse the difference between being-in-itself and being-for-itself, Sartre would communicate in parables. One of his more succinct tales, which I read in a work called Existentialism and Human Emotions, was the story of a student of his who came to him during World War II, and asked him for help with a problem. The student lived with his mother, who was somewhat aged and in poor spirits, and whose sole comfort was the presence of her son. But the student also was inspired by the fight against Fascism, and wanted to make the long trek through Spain and on to England to join the Free French. Sartre reports telling the student (I imagine that the incident may have been a composite, and that Sartre may be editing it for literary effect) that there was nothing that he, Sartre could say. The young man was forced to choose between these two options, and there was nothing that Sartre or any other authority could tell him about his dilemma.
What I find most interesting about this is that Sartre lays out the case for both of the boys options quite sympathetically. He acknowledges the importance of the family loyalty. He also admits that the boy's sense of national loyalty and opposition to fascism are important. He goes through a list of practicalities involved in the decisions: does it make sense to take great risks to join the opposition, when the student might be detained in Spain? Sartre actually shows a decidedly pragmatic turn when discussing these things, which I thought would bring shame to any good existentialist or Frenchman. But good pragmatic advice doesn't equal good philosophy. If his point is that man is forced to be free, he undermines it by just pointing out what I thought that everyone already knew: that life is complicated, and that there are often decisions where both courses have good, and bad things about them. Sartre's big revelation seems to be that there is no magic book, written either in ether or gold leaf, that can tell us what to do and after which we wouldn't have to feel any worry, doubt or regret.
This shows also in the fact that Sartre only discusses the two options that make the most sense. Sartre does not suggest that the student should become a collaborator, either out of support for fascism, greed, or sheer murderousness. Sartre does not also suggest that the student descend into absurdity, going out into the street and flinging feces. Neither does Sartre suggest that the student descend into quietism. Sartre takes the situation as presented as a real situation, and deals with it seriously. It is only in the solution to the situation that he says there is no final guide.
Sartre's assertion that Kantian ethics is helpless in this situation shows me that like perhaps most of the mainstream of European thought, he misunderstands Immanuel Kant. If youth is wasted on the young, Immanuel Kant's thought is wasted on Kantian thinkers. I don't believe that Immanuel Kant was a dogmatist, and even if he was, the categorical imperative is not an external rule to be discovered. Immanuel Kant didn't want to do away with human judgment. "People who can make a meaningful contribution, and who are not immediately required to help others, should take risks to make that contribution" works as a categorical imperative, after which it is up to the student to decide both his ability to actually help the resistance, and the immediacy of his mother's dependence on him. So I don't know how radical of a step Jean-Paul Sartre has taken away from the traditional corpus of ethical thought in talking about this student and his plight.
So while it is helpful that Sartre can present his philosophy in an anecdote that is both dramatic and meaningful, I also do not think that this anecdote shows that Sartre's philosophy of ethics, and philosophy of existentialism, has made that radical of a leap forward. All Sartre seems to be telling us with this anecdote is that making decisions can be very hard. And we all already knew that.