There’s a scene in The Avengers where Black Widow is fighting Hawkeye. He’s being mindcontrolled by Loki and is trying to kill her; she just wants to incapacitate him. A few minutes before this fight scene, she hurt her ankle and was visibly limping when she was fleeing the Hulk. She volunteered to go after Hawkeye, despite being understandably shaken by her encounter, and the fact that she and Hawkeye clearly have an emotional bond.
Oh, and the fight? She won. And she didn’t win because Hawkeye was distracted or something happened that got his attention or he was wounded from a previous scuffle. She won because she’s better. There was no reason for her not to—Hawkeye is an archer, not a hand-to-hand expert. If it had been a contest of marksmanship, he would have won, and rightly so. But it wasn’t, and she won by simply being a better fighter.
The best part, though, was probably when he looked up at her, after being hit in the head, and said “Tasha?” A lot of people would have stopped fighting then—I probably would have done—but Natasha Romanoff is a trained spy, and she just clocks him in the head again. It’s the choice of someone whose job is to be suspicious of other people, regardless of what they might mean to her.
This sums up a huge part of why I love Black Widow—Natasha Romanoff—so much: she is a professional and she acts like one.