Your characters are thinking all the time. You want to filter out all but their most important thoughts to convey to your reader.
Those thoughts should be the ones that will specifically move your reader’s emotions.
Thoughts are related to the writing craft topic of point of view. If you get deep into your character’s point of view, then his/her thoughts enhance the scene emotionally.
The key here is that your character’s thoughts tell the reader how the character feels about the events happening, other people, or the surrounding area.
Compare these two:
Andrea O’Malley paused on the threshold of the Chinese restaurant. She wasn’t sure if she liked the exotic smells that teased her nose—spices she couldn’t name, as well as nutty sesame oil, salty-sweet oyster sauce, pungent soy sauce. She patted her French twist, which didn’t need fiddling with. She couldn’t help it—she was a golden-haired alien in the midst of these black-haired party guests. At least she hadn’t dressed inappropriately—the other guests stood talking in clusters, the women in short silk dresses like her own.
Lex Sakai raced through the open doorway to the Chinese restaurant, immediately immersed in conversation, babies’ wails, clashing perfumes and stale sesame oil. She tripped over the threshold and almost turned her ankle. Stupid pumps. Man, she hated wearing heels.
The reader gets two very different descriptions of the same party, but through different characters’ thoughts. It not only describes the scene, it reveals things about each character, the character’s emotions, and the conflict to expect in the scene.
Andrea is elegant but uneasy because she feels out of place. Lex is in very familiar surroundings, but impatient about being there.
So be choosy about what your character’s thoughts are. Make their thoughts reflect the emotions of your character, and evoke emotion from your reader.