Others have nailed it. It's the casting that makes this movie interesting. Makes it worth watching too. Many names here. Ironically, Harrison Ford, probably the biggest name of all when one takes the long view, was an absolute total no-named nobody in 1967. Glenn Ford was the only true Hollywood movie star in the cast, although probably a little past his prime at this point. Meanwhile, Paul Peterson, Inger Stevens, and even Max Baer, Jr, who were household names in 1967, might well have younger folks these days scratching their heads, saying "Who?" But they were names then, mainly TV names of the day, but names nevertheless.
Based on the inspired casting, clearly somebody had some higher aspirations for this movie. Somebody was trying hard to inject superior production values into this project. Somebody wanted this to be a box office success, maybe even a noteworthy film. But, alas, whatever it was, something was lost along the way. We could speculate about it 41 years later, try to pin it on somebody, but why? No point to that. Suffice it to say that somehow somewhere before all was said and done it lost its edge.
Another consideration is the year, 1967. How could this offering ever hope to compete? As I've written elsewhere, 1967 was the very best year ever for movies. The Graduate, Cool Hand Luke, In The Heat of the Night, Guess Who's Coming to Dinner, Bonnie and Clyde, The Dirty Dozen. Remarkable films all. There might be one such notable movie of the caliber of those in any one year. Two would be better than average. But six in one year? Extraordinary indeed.
The point is that 1967 was a remarkably good year for movies. Of course it's hard to flatly state that it was the very best movie year ever, because how could one possibly measure that? It is based on pure opinion. But try this: name another year that was any better than 1967. No can do. So this is the stuff A Time For Killing was up against as competition for the box office dollar back in 1967. It never really had much of a chance. In another year it might have fared a little better. But in 1967 it got lost.
