IIRC, Belzer had terrible auditions for L&O. So bad that Belzer himself didn’t want to do the show.

But the producers loved him and believed in him and pushed him to take the role. I think it’s cool that he had that much longevity after bombing so hard from the start.

ETA: "Barry Levinson, executive director of Homicide, said Belzer was a "lousy actor" during his audition when he first read lines from the script for "Gone for Goode", the first episode in the series. Levinson asked Belzer to take some time to reread and practice the material, then come back and read it again. During his second reading, Levinson said Belzer was "still terrible", but that the actor eventually found confidence in his performance."