Fear

Fear was an emotion. The Twelfth Doctor saw fear as a superpower. (TV: Listen) He further told Bill Potts and Nardole that "Fear keeps you fast. Fast is good." (TV: Oxygen) Due to the effects of adrenaline, (TV: Oxygen) "your heart is beating so hard, I can feel it through your hands. There's so much blood and oxygen pumping through your brain, it's like rocket fuel. Right now, you could run faster and you could fight harder, you could jump higher than ever in your life. And you are so alert, it's like you can slow down time." (TV: Listen) Such a feeling would result in faster breathing, as well, though that could lead to a faster death, in places where oxygen was in short supply. (TV: Oxygen)

Clara Oswald expressed, to a young First Doctor, that "fear can make you faster and cleverer and stronger. [...] If you're very wise and very strong, fear doesn't have to make you cruel or cowardly. Fear can make you strong." She concluded that fear was like a companion, a constant companion, bringing people together. In fact, "Fear makes companions of us all." (TV: Listen)

The Twelfth Doctor sometimes purposely scared his companions, in order to "max up" their adrenaline, and therefore their capability at solving and surviving a new situation. (TV: Oxygen)

According to Miss Quill, the "first fear" was a fear that "everyone" went back to and could not face. The Lorr Ballon claimed that this was just cowardice. Quill claimed that a soldier without fear was inefficient, and was one that won battles, but lost wars. (TV: The Metaphysical Engine, or What Quill Did)

Some beings could feed on fear, like the entity on Phobos (AUDIO: Phobos) and the Fearmonger. (AUDIO: The Fearmonger)

The Keller Machine fed off the fears of its victims. They included Arthur Linwood, who was afraid of rats; Kettering, who was afraid of drowning; the Third Doctor, who was afraid of fire; Alcott, who was afraid of dragons; and, who was afraid of the Doctor. (TV: The Mind of Evil)

Permians hunted by inducing a state of fear in the minds of their prey. (AUDIO: The Land of the Dead)

The First Doctor noted fear as an example of emotions to the Cyberman leader Krail, questioning "Have you no emotions, sir?" (TV: The Tenth Planet, Twice Upon a Time)

The Sixth Doctor listed vertigo with agoraphobia and xenophobia as fears which, when mixed together, would form "naked fear". (PROSE: The Ultimate Evil)

When facing Eknodine, the Eleventh Doctor remarked to Rory Williams that they were bent on killing them as fear turned them into savages. (TV: Amy's Choice)

Confronting the Daleks, the Ninth Doctor voiced his belief that, despite their purging of emotions, one little spark of fear remained in their DNA. He goaded to them that that spark of fear "burn[ed]" when they faced him. (TV: The Parting of the Ways)

Addressing humankind shortly before the Battle of Canary Wharf, Cyber-Leader One assured the people of Earth that they need not fear as the emotion would be removed upon their conversion into Cybermen. (TV: Doomsday)

George psychically deposited all his fears in his doll's house, which manifested as peg dolls. (TV: Night Terrors)

On the Minotaur's prison ship were pictures of those who had fed it and what their fear was. They included Novice Prin, who was afraid of sabrewolves; Lady Silver Tear, who was afraid of Daleks; Royston Luke Gold, who was afraid of Plymouth; Tim Heath, who was afraid of having his picture taken and Tim Nelson, who was afraid of balloons. (TV: The God Complex)

The Seventh Doctor regarded fear as a self-preservation mechanism. (AUDIO: Harvest of the Sycorax)

The Eleventh Doctor thought that anyone who wasn't scared of a Weeping Angel was a moron. (TV: The Time of Angels)