Because of the heavy philosophical element of Unbearable Lightness, much of what we know about the characters comes from the narrator's exploration of what they think. Consider, for example, Part 4 of the novel, aptly titled "Soul and Body." Tereza's character is very much defined by the way she views her soul as something separate from her body, and by her often antagonistic relationship to that body.
Similarly, consider what we learn about Franz and Sabina from the "Dictionary of Misunderstood Words," a catalogue of their conflicting thoughts and opinions on various topics. Along the same lines, Tomas's own musings on es muss sein and eternal return provide a wealth of insight into his character.
The themes and philosophy of Unbearable Lightness is so tied up in sex and love, that it's impossible to talk about one without delving into the other. The way that the various characters view sex and act sexually tells us a fair amount about who they are, particularly in relation to the novel's philosophic interests.
For an example, think about Tomas's womanizing. The narrator goes into the details of Tomas's particular brand of womanizing – epic and not lyrical – and the reason he is compelled to do it. It turns out to have to do with es muss sein, one of the novel's central philosophical themes. His behavior represents Tomas's curiosity and his desire to take control and possess. You could also think about Tereza's inability to have casual sex with the engineer. This isn't just about sex – it's about Tereza's inability to live with lightness. Again, sex brings us right back to one of the novel's central philosophic veins.
We're most interested in Tomas's job as a surgeon and Sabina's career as a painter, because these get particular attention from our narrator. The narrator relates Tomas's job as a surgeon to his incessant womanizing. Both are part of his es muss sein, and are beyond his control. Both are also rooted in Tomas's native curiosity. When Tomas imagines having sex with a woman, he's using an imaginary scalpel. He wants to see what is hidden, and he wants to take possession of something that it not accessible to others – just like surgery. In this way, Tomas's occupation reveals a good deal about his character.
Something similar is going on with Sabina – her paintings reveal her way of looking at the world: "On the surface, an intelligible lie; underneath, the unintelligible truth showing through" (1.11.1). Her profession as a painter has a lot to do with Sabina's reaction to kitsch, a reaction that defines her character and drives the string of betrayals that constitute her life choices. For more on kitsch, paintings, and Sabina's character, see her "Character Analysis."