Short
Uhura turned her back in disgust toward the man seated next to her at the bar. She felt a hand on her shoulder and turned back only to see the man was not going to take no for an answer. Uhura deliberately plucked his hand off her shoulder and dropped it. The man growled at her. From behind, Uhura heard a familiar Scottish brogue say "Doncha think ya ought to leave the lady alone?" Scotty appeared next to Uhura and gave the man a glare. The man narrowed his eyes and stood. Scotty gulped as he stood nose-to-chest with the man.
A/N: I guess I stole this scenario from myself. I just get a kick out of Scotty standing up for a lady and suddenly finding himself up against someone much bigger than him.
Stain
Chekov abruptly stood and walked out of the rec room. He wandered the corridors of the ship, gazing at his hands now and then, occasionally muttering to himself.
Captain Kirk was striding purposefully toward the turbolift when he found Chekov leaning against a wall. Kirk gazed at his young navigator sympathetically. He knew all too well the emotions swirling through the ensign's mind. Regardless of the circumstances, a man's first kill left an indelible stain on mind and heart. Abandoning his current task, Kirk gently took Chekov by the arm and led him back to his own cabin for a drink.
A/N: Thanks to Satin Ragdoll for the prompt.