Hi... It's now Wed 5/23... This is an edited list of sorts... forgive me if there are some repeats.
Starting with, and in no particuluar order:
Ron Halder (Cronus on Stargate SG-1 ep. Fair Game, and others) in the MacGyver episode Blind Faith, In the beginning of the episode, when Mac meets Pete at the train station, after the courier is murdered, he is one of the police officers.
Jay Brazeau. On Stargate SG-1, he was Harlan from Tin Man. On MacGyver, he was first in Kill Zone - season 3, as the military guy Colonel (?) Phelps. Then he appeared in The Gun - as Zamora - the gun dealer who wants that special gun.
Jerry Wasserman. On Stargate, he was the secretive guy Whitlow that General Hammond meets up with in Touchstone. On MacGyver, he appeared in Collision Course as Falco, Live and Learn as Nick Milani, the father of Tony (remember, he was caught when the building collapsed and Mac had to get him out.) He was also in the episode The Gun as Maddox, the guy supplying the guns to Zamora.
Garwin Sanford - Narim of the Tollan on Stargate. On Mac, he played Durst (the murdered boyfriend of Katie - with the baby) and he also was Raphael from On A Wing And A Prayer.
Shane Meier. On Stargate, he was Garan in A Hundred Days. On Mac, he was in Harry's will as young Mac. Also Passages as the boy Harry talks to on the dock while fishing.
Marie Stillin. On Stargate, she played the Tollan Chancellor Travell. On MacGyver, she was in There But For The Grace as Rachel, one of the church ladies.
Blu Mankuma. On Stargate, he was Sheriff Knox in the creepy village (filmed in Steveston, Richmond, BC) in the episode Nightwalkers. On MacGyver, he was in Hearts of Steel and There But For The Grace as Lieutenent Rhome, and before that he was in Invisible Killer as Sheriff Max Hubbard (he's the sheriff type, I guess)
Tracy Westerholm. In the Mac episode The Black Corsage as "The Blond", and in many, many SG-1 episodes as one of the gate technicians/SFs. I think she was even pregnant in one episode.
Garry Chalk - General Chekov on Stargate, and in the Mac episodes The Ten Percent Solution as Sergeant Grey (police guy) and In Thin Ice as Tony Ellis the recruiter guy and in Deadly Dreams as Sweeny.
Terry David Mulligan was Danny Barrett (Nikki's brother) in Fire & Ice and Frank Rogan in Two Times Trouble on Mac. On SG-1 he was David Swift (defense secretary?) at the beginning of The Nox.
Paul Koslo was the scary Dr. Suvarin in Prisoner of Conscience on Mac. On SG-1 he was the even scarier Terok - the guy who tortures Teal'c in The Serpent's Venom.
Steven Williams was Charlie Robinson in Countdown on Mac. On Stargate he was General Virdrine in Absolute Power and Tangent.
Byron Chief Moon was Joe Wales in Ghost Ship on Mac. On SG-1 he was Elder #2 in Spirits.
Chief Leonard (Len) George was Dave Wales in Ghost Ship on Mac. He was Elder #3 in Spirits on SG-1.
Robin Mossley was Wrigley in Jack in the Box, Earl in The Endangered, Alexander Shannon in Legend of the Holy Rose (pt.1), Charity Santa Clause in The Madonna, Wilt (sometimes Bozer) in Serenity, The Lost Amedeus and MacGyver's Women - all on Mac. And of course he was Malakai in Window of Opportunity on SG-1. (whew!!)
Bruce Harwood was Juice in Blowout and Willis in The Wall, The Visitor, The Wasteland, Trail of Tears and Hindsight. On SG-1 he was Dr. Osbourne in Frozen.
Charles Payne was Breeze in The Challenge, The Madonna and The Gun on Mac. On SG-1 he played and SF/Airman at SGC in Cold Lazarus.
Marshall Teague was also in MacGyver's Women as Kid Curry as well as being Steve Morrison in Renegade on Mac, and of course, Colonel Cromwell in A Matter of Time on SG-1.
Henry Gibson was also in Harry's Will as "Chicken Habit Customer" as well as playing Pinky in Deadly Silents on Mac. He was Marul in the Sentinel on SG-1.
I think Dan Shea, Tamsin Kelsey, Peter Williams, John De Lancie, Bill Dow and Don Davis were already discussed. Whew! Stayed up until 2 in the morning cross-referencing all these actors. Bravo to them!
That which has been established will endure even with time and at a distance.
I can't tell you anything about what happens after the cameras go off, mostly because I'm usually unconscious by then.