James Garner a real man, yes. Steve McQueen? No even close--
On The Great Escape McQueen was being such his usual pain in the ass and taking too many long breaks from the filming, the producers and director John Sturges started giving more and more of his lines to Garner's character.
Finally seeing the writing on the wall, old Steve came back and finished filming.
On McQueen's next to last film, Tom Horn, I was told he kept telling the producers and screenwriter he wanted a scene where his character sat around a campfire and talked about the good old days. Told there was such a scene, he said he wanted one where the old days were better and his character said less. This went on completely all through filming and I understand they filmed some seven different versions. The final one having McQueen just nod to comments by the other actors.
None of those seven scenes... 
were used in the final cut--