Data as Frank Hollander
Frank Hollander was a holographic character in a holoprogram set within the Ancient West town of Deadwood, which Alexander Rozhenko created for himself and his father, Worf, in 2369. He was the father of Eli Hollander, another fictional character from Deadwood.
After Worf jailed Eli while playing the sheriff of Deadwood, Frank kidnapped Alexander. He offered to release him in exchange for Eli. Due to a malfunction, Data assumed the identity of the holo-characters, and the safeguards were shut down, leaving Worf and Alexander in real danger. During the exchange of prisoners, Worf beat Frank in a gunfight, ending the program. (TNG: "A Fistful of Datas")
Frank Hollander was played by Brent Spiner. We never actually see the "real" Hollander since when the character first appears in the holodeck program, the malfunction has already caused him to appear as Data.