In "The Third Level," Sam is a friend of the protagonist, Charley, who works as a psychiatrist.
Firstly, Sam is intelligent and knowledgeable in his field, as evidenced by his ability to diagnose Charley's condition accurately and his understanding of the psychological mechanisms behind his obsession. For instance, he says to Charley, "Your Third Level, in short, is a form of escape, of wish fulfillment, from the stresses and problems of your life and your times." This shows that Sam has a good grasp of Charley's situation and is able to explain it in a clear and concise manner.
Another quality of Sam's character is his skepticism. Throughout the story, Sam maintains a skeptical and rational attitude toward Charley's claims of having discovered a portal to the past. He questions Charley about his experiences and suggests that they may be a product of his own psychological state, rather than an actual time-traveling adventure. However, at the end of the story, the reader is told that Sam has found the third level and traveled back in time himself.