His casting was probably due to his heritage and his looks.
He is part Native American. If you are old enough to remember, there was a significant increase in casting of Native Americans in dramas in the early 70's. Not just Emergency as a permanent series cast member, but in episodes of McMillin and Wife, Quincy, McCloud, Streets of S.F., Kung Fu, Columbo, etc. etc. etc. I'm guessing this casting was partly due to increased awareness of Native American civil rights movement that eventually climaxed in 73 at Wounded Knee (show started in 72) and awareness via children through Sesame Street's increased number of N.A. kids and cultural segments, along with heavy doses of N.A. culture, skills, etc. in Boy and Cub Scout organizations in the 60's and into the 70's.
The show needed cast members to appeal to the family demographic (Roy was married with children) and women (Johnny was single). The male demographic along with children was covered by the action scenes, fire apparatus, etc. Mantooth's photos got good results when shown to women. His acting did get a little better as the show matured, but it was never close to the two real actors in the cast. ** His character is supposed to be a bit irritating, so it makes his acting seem worse.
**Robert Fuller (Dr. Brackett) had been in several TV western series and Julie London (Dixie) was a long time professional singer and movie actor. Bobby Troup (Dr. Early) was a professional musician, famous for writing "Route 66". His delivery was worse than Mantooth's. If you want his character to be ruined for you, listen and you will notice it sounds like he's sitting in a grade school reading class reading aloud in class.
The rest of the cast with the exception of Mike Stoker had all been bit actors and extras in other shows, mainly in shows produced by Jack Web. Mike Stoker (real name) was a career firefighter and eventually retired as Captain. Tim Donnaly (Chet) was a stoner on Adam-12 and Dragnet. His brother Dennis was a stock director for Webb and directed a few episodes of Emergency. Mantooth's brother was in an episode or two.
All that said, Emergency! has that cult following thing going for it. With all it's lumps, it's very addictive, especially for me. I was a very impressionable 6 year old when it debuted. Watching it brings back awesome childhood memories!
