Shatner's performance.

I've seen William Shatner in a lot of films, plus Star Trek & T J Hooker, and in none of them was he as good as this, not even his good comic performance in Third Rock from the Sun. This was a wonderful surprise that he was capable of this standard so early in his career, and then apparently went on to waste it.
His next best was many years later when he was surprisingly good in 'A Christmas Horror Story'.

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That's hard for me to believe that Corman would actually think that.

In my opinion, this is Shatner's best performance.