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Jeffrey Donovan comes from an impoverished family and has twenty years of martial arts experience (including a black belt in karate).
Also, in "Fast Friends," Sam mentions that he once got to the "Michigan State Quarterfinals" with his arm (shot put? football?); Bruce Campbell is originally from Michigan (the Detroit suburb of Royal Oak, to be exact).
Sam and Fiona approach a group of con artists while under the guise of federal agents who had been monitoring them. Sam introduces himself and Fi as detectives Cagney & Lacey, alluding to costar Sharon Gless.
Additionally, in the second season Fiona starts dating a man named Campbell. In one episode, a conversation between Fiona and Sam takes place, with Fiona commenting on how good-looking a particular man is, to which Sam (played by Bruce Campbell) replies, "Well, he's no Campbell."
Michael also makes contact with a Libyan operative named Anwar in one episode.
Inverted in a "Signals and Codes" where the client of the week was played by an actor named Michael Weston.
In one episode, Sam and Barry are discussing people who only use one name, and Sam says "Spider-Man" as an example. Bruce Campbell has a cameo in all three of the Spider-Man movies.
And in the winter premiere of season 3, Tyne Daly guest stars.
Word of God mentions that as a general rule, they write characters with a specific actor in mind. This is in part due to the need to quickly establish characters and chemistry (see the above Tyne Daly instance) as it is just having fun.
There was also "False Flag", where Fi said Sam's bulletproof vest smells like Old Spice and bourbon. Bruce Campbell is a spokesperson for Old Spice.
In one episode, Sam is doing surveillance on Carla, who takes time to swim. Commenting on her ability, Sam says: "The woman is a machine." Carla's actress, Tricia Helfer, played a robot on Battlestar Galactica.
In 5x6, Sam compliments Mike on his chin. Mike instantly turns to face Sam, seemingly very surprised, and notes that it means a lot coming from Sam. Bruce Campbell (Sam) really is known for his chin among other things — he even titled the book he authored after his chin.
A recent episode showed Gabrielle Anwar once again getting to tango.
John C. McGinley plays Michael's former CIA mentor Tom Card. References to McGinley's role as Dr. Cox on Scrubs abound: among other things, Card displays expasperation and impatience with his young protege, he does a ton of Deadpan Snarking, and he even does Dr. Cox's frustrated whistle in one scene.
Barry the money launderer refers to Sam's shotgun as his "boomstick."
Tom Baldwin gets scammed out of a painting by Michael in "Seek and Destroy."
It seems Toby Ziegler snuck out of The West Wing for half an episode.
They seem to be running through Jeffrey Donovan's old The Pretender co-stars. Patrick Bauchau (Sydney) plays Lucien the bombmaker in "Eye for an Eye", and Michael T. Weiss (Jarod) plays the lead bank robber in "Army of One".
Sergeant James Doakes makes a brief appearance as one of Michael's temporary meeting facilitators in Season 3. Mike even injects him with a syringe Dexter-style to make him think he's been poisoned.
Dead Larry is, depending on your age, Vice President Hoynes or Otter. Nerdy LAPD Detective Dutch teamed up with him in the penultimate episode of Season 4.
Harry Dresden comes calling as a terrorist leader bent on capturing Fiona and dragging her back to Ireland. Needless to say, Michael makes sure it doesn't end well for him.
Simon is Cromartie and John Henry! The scene with him slow-motion walking towards an injured Michael in a police uniform with a pistol gripped at his side is a clear Shout-Out. The show seems to have a habit of hiring former Terminators on.
Vaughn is Uriel and Bunny Colvin. And speaking of Colvin, a bunch of others from The Wire also show up. Rawls became a Congressman from Florida, Ziggy apparently escaped from jail only to become a Mad Bomber, Dennis "Cutty" Wise and Kima are now in the CIA, and McNulty's ex Elena from from being a real estate agent to a Classy Cat-Burglar who pretends to be a real estate agent in her first appearance.
And Some Other Stuff: In "Lesser Evil." Michael's explosive recipes contain just enough to tantalize MythBusters and brush over the rest to avoid liabilities. Usually. Sometimes it's just to handwave that an explosion is a bit larger than should be expected.