Mr. K

Mr. K (Erik Crary) is a man who sits alone in a dark office. He might be an incarnation of Jack (Jack is seen sitting down at his desk, and seconds later, Mr. K has taken his place). Sue visits him while Doris is trying to kill her, claiming that she was told he could help her. She then talks for a long time, telling him many stories about her life. Mr. K listens passively with no reaction whatsoever to Sue's stories of violence and trauma. The only time he interjects is when Sue tells a story about her boyfriend attacking her with a crowbar because he was jealous that she was cheating. Mr. K asks, "Were you in fact seeing another man?"

Mr. K lets the phone ring for a long time (as Jack does), but eventually goes to a back room and answers it. He tells the person on the other end: "Hello? Yeah. She’s still here. I don’t think it will be too much longer now. Yeah. The horse to the well. Yeah. Crimp? Yeah. He’s around here someplace, that’s for sure." This seems to indicate that Mr. K is one of the individuals interested in flushing out the Phantom and leading Nikki/Sue to kill him. The phrase "horse to the well" was also spoken by Darek, another incarnation of Jack Rabbit. Sue flees the office while Mr. K is on the phone.

After Nikki leaves the soundstage, she is strangely transported to a supernatural movie theater. Mr. K appears, walking through the theater. He stops at the bottom of a staircase and stares at Nikki for a long time, then walks upstairs. Nikki follows him upstairs, and finds the gun that she uses to kill the Phantom.

More Things That Happened (Deleted Scenes)
The deleted scenes contain much more footage of Mr. K listening to Sue's monologue.

Analysis

 * When Sue is first seen walking upstairs to Mr. K's office, a snowy Polish street is visible out the window behind her. The second time we see this scene play out, Mr. K's office is above a club in Los Angeles, and the window in the staircase is gone.
 * The only thing Mr. K says to Sue is, "Were you in fact seeing another man?" In the deleted scenes, he says one more thing to her. When she talks about her mother's hand being chopped off in a machine and says the hand was "flatter'n a fuckin' pancake," Mr. K chimes in: "I like pancakes."
 * Mr. K has a rolling pin on his desk, and what might be garden shears hanging on the wall. According to Erik Crary, he has a paint scraper in his pocket, and Lynch chose his glasses from a selection of Crary's old pairs. (Source: Room to Dream, David Lynch and Kristine McKenna, 2018, Random House, p. 416.) 
 * Sue's monologue in his office was the first sequence scripted and shot for the film. It was originally intended to be an Internet short, and ended up being seventy pages long. (Source: Catching the Big Fish, David Lynch, 2006, Penguin Group, p. 141.)
 * Erik Crary began working as Lynch’s assistant in September 2000. (Source: Room to Dream, David Lynch and Kristine McKenna, 2018, Random House, pp. 411-12.) He is an associate producer on the film, as well as production supervisor, camera operator, and "production team." He was also credited as assistant to Mary Sweeney and Neal Edelstein on Mulholland Drive.