What was Martin Castillo's background before he became a vice cop?
He worked for the DEA for many years as an undercover agent in southeast asia. In the season 1, two part
episode, The Golden Triangle, Castillo tells Tubbs how he once was stationed in the mountain range in between Burma (now Myanmar) and Thailand for three years. Tubbs says, "three years, that's a long time." To which his boss replies, "its a lifetime." And in the season 3 episode, The Savage , there's a detailed flashback to Castillo's past as an agent once stationed in southeast asia during the Vietnam war in the early 1970's. He was investigating the murders of several local prostitutes with the help of a local police investigator name of Tran. Who came to Miami in the present searching for the same serial killer from the past. It seems that Martin Castillo certainly has had a ton of on the job experience as both an investigator and drug enforcement agent. He's an old school by the book cop who's been there, done that.
What happened at the end of the series?
At the end of two-parter 'Freefall/World of Trouble' we see Crockett and Tubbs resign from the police force, Tubbs planning to head back to New York to be a Private Investigator and Crockett saying that he's going to just keep heading south in Florida and see what happens (it's possible that he's actually rich, his dead wife presumably having left him her earnings from pop stardom). As Crockett drives Tubbs to the airport he asks him has he ever considered a career in southern law enforcement just as he does in the pilot. It seems that the series is at an end but there are then three more episodes after that. It's possible that they actually changed their minds and decided to go back to work for Metro Dade Police again.
Was Don Johnson the first actor cast to play Sonny Crockett?
No, in fact in 1984 prior to casting the pivotal roles of Crockett and Tubbs then NBC entertainment chief
Brandon Tartikoff had considered former Chips actor Larry Wilcox for the co lead. One reason was that there was a shortage of leading men in the mid-30's age group and that Wilcox had been a cop on the previous television series. However executive producer Michael Mann and creator/writer Anthony Yerkovich had remembered Don Johnson from four failed pilots and had him audition with Philip Michael Thomas for the lead roles. Therefore Wilcox was out and Johnson and Thomas displayed strong on camera chemistry so were then cast as the leads and the rest became prime time television history.