He was definitely pretty good in it.

It took me a few moments to recognize him he was so young.
He was excellent.
But so was everyone else in this film. The acting is first-rate. After watching this I wondered if anyone did get an Oscar nomination. Nope. Not a single nomination in any category.
But Julie Andrews won for Mary Poppins? Unbelievable. I love Julie Andrews but that performance was not Oscar worthy at all.
And Fail-Safe is still available on Tubi.

Agreed. Hagman was indeed excellent as Buck.
You've seen Guy Standeven in something because the man was in everything.

I thought he was excellent as well. I wonder if he was intimidated by the presence of Fonda, which translated into the character's nervousness due to the situation he was in.
I disagree with what you are saying, but I will fight to your death for your right to say it;-)

And now he's gone.

It's a great film, but Hagman DOES employ a slight Russian accent, which is distracting.

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What Russian accent? He made no attempt at one in this movie that I am aware of. Are you sure you're not thinking about a different movie?
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You must be talking about another film! This one is a taut thriller and far from tediousalthough I'll grant you that since it has no chase or fight scenes it may not be to the taste of those who prefer James Bond or Jason Bourne movies.

Are you kidding? He stunk up the place with that ridiculous and unnecessary Russian accent. I find his performance here contrived and poorly directed.
This is an overall tedious film and not worthy of all the high powered talent that lent their time to it. (Fonda, Weaver, Matthau, etc)