I chanced upon an article about Jack and Stoney Burke that was written by Robert Dowdell, who portrayed Stoney's sidekick Cody Bristol. In the article, entitled "The Stoney Burke I Knew," Bob Dowdell spoke very highly of Jack and explained many of the issues for which the naysayers faulted Jack through the years.
Yes, Jack kept himself to himself, but he used that time to study his lines. After filming ended for the day, he studied his lines on the way home, and he went home to Marie, rather than out to drink with the bachelor members of the cast. Nothing bad about that!
Mr. Dowdell's ability to put his own professional ego aside long enough to tell the truth speaks exceedingly well for him. Such a shame it was that another cast member, who shall remain nameless, wasn't able to do the same thing.