The constant mantra out there among writers is “show, don't tell.” That is, don't just tell the reader that a spaceship feels cramped, show it in the narrative by the crew members ducking to avoid overhead power lines, stepping carefully to avoid stairway alcoves, and almost colliding with each other. Don’t tell us that a man is almost drowning on the coast of the Ionian Sea, show him hanging on to a jagged half-submerged rock, and then the pounding roar of the surf headed towards him. And above all, don’t tell us that a character is smart, brave, pious, seductive, etc., but show us by the character’s actions and how others respond. This makes the writer give vivid descriptions that the reader will find convincing.
Unfortunately, this is a vicious half-truth.
The advantage of books over TV or movies is we can describe what the characters think and feel. And the way to do this is to tell the reader. I've tried to show thoughts and feelings by action, which doesn't work. For instance, I tried to show the desperation of the man in the Ionian Sea by his getting weaker as the rough surf pounded him. And I thought I was showing how ingenious he was in his how he acted to save himself. The criticism I received from online critiques indicated I didn’t show his thoughts or feelings.
Tell the reader what the character is thinking, what the character is feeling. Hopefully, you can avoid trite phrases such as “He thought,” or “He felt.” There has to be good cause and effect, linked to the physical world. The character may observe a certain detail, which leads her to think she’s being used as bait, or more explicitly, Using me as . . . bait. Or the character’s feelings may be linked to a clenched stomach or tense shoulders, or suspicion that his eyelid is twitching.
Show the reader your character and surroundings by showing, not telling. But plunge your reader into your character’s thoughts and feelings by telling what they are.