Robert Picardo played the Emergency Medical Hologram on Star Trek: Voyager for seven seasons and has since returned to the character on Star Trek: Prodigy. He'll also portray the character yet again on the upcoming Starfleet Academy. After so many episodes and seasons, he knows the doctor inside and out. But, of course, that wasn't the case when he first saw the doctor on script, then named Dr. Zimmerman.
According to an interview in Star Trek Communicator during Voyager's first season, Picardo just didn't get the character. He thought playing a "machine-type guy with no personality" would get very old very quickly. In fact, when he saw the audition lines for the doctor, the description read "colorless, humorless." Picardo passed on that audition and, instead, auditioned for the part of Neelix, which ended up going to Ethan Phillips.
Voyager producers came back to Picardo and asked him to reconsider the part of the doctor because they'd liked what he'd done during his Neelix audition. Picardo was still uncertain because he didn't know what could be done with a machine character. But he auditioned anyway and was cast for the part.
Still, he didn't know what to do with the character and admitted he "faked his way through the pilot" because he "didn't have a clue exactly what to do with it [the role], aside from a basic attitude." But when he got the script for the second episode, "Parallalx," things fell into place, and he said he got it, realizing the part was all about this attitude. He was sarcastic and unapologetic about it, and it worked with the viewers.
Over the years, the Doctor has given us some laughs, and we couldn't imagine anyone else playing the part. After the first episode, Picardo started getting excited about the part, and it showed. He took a character that could have remained "colorless" and "humorless" and made him into one of the best parts of Voyager. He was easily one of the best parts of the second season of Prodigy as well. He alone could make Starfleet Academy worth the watch!