redford: the klingons are right behind us, we've got no place to go.
newman: we'll jump!
redford: into a wormhole?
newman: sure, they'll never follow. Would you jump into a wormhole if you didn't have to?
redford: I do have to, and i still won't do it. I want to fight them.
redford: I can't spacewalk. I get sick ...
newman: Hell, the gravitational gradient'll probably kill you!
newman&redford together: woooo-ohhhh!
as they fall into an oscilloscope pattern, leaving their bumpy headed pursuers (played by Strother Martin and Henry Jones) far behind.
Naah, I just don't see it ... plus, as I recall, the apocryphal story of recasting according to a facetious Susan Sackett and GR was Redford as Spock and Clint Walker (!?) as Kirk.
The Newman/Shatner issue IS interesting, since after James Dean died, Newman got all the parts Shatner was up for, including CAT ON A HOT TIN ROOF, which the latter was considered pretty much a shoo-in for.