With respect to older actors: just after Reagan left office, he was on some TV talk show (maybe Larry King?) and was asked what question was he asked the most often. Reagan said that he was most often asked about acting with Errol Flynn (even more than Iran-Contra).
Reagan also pointed out that the old character actors could steal a scene better than younger stars. Francis X. Bushman's performance in "Sabrina" is an example (as are most performances by Jack Elam or Walter Brennan.) John Wayne made a career out of letting (helping actually) other actors steal scenes. James Dean was coached on how to steal the scene in Giant where he is given the ranch by Mercedes McCambridge.
Best recent example was the one scene in Troy containing both Brad Pit & Peter O'Toole. Made Pit look like he had a speech impediment.
The wild stochastic variable
Is loose, and vague, and quite unstable
He's so imperfectly located
It's quite a job to get him mated.
(no offense intended, but I couldn't resist.)
Garner and Rowlands didn't just steal a scene, they stole the whole movie. Quite a feat considering they were up against youth and beauty. And all they had was about a hundred years of show biz experience between them...
Dean, in my opinion, was just an idiot. Not a great actor and probably would have gone into obscurity if it wasn't for a guy named Turnipseed (how's that for hollywood trivia?).
If you want to see some great attempts at scene stealing, check out Magnificent Seven.