The fact that Shatner himself portrayed Kirk in so many different ways only adds to the dilemma. Kirk's been everything from square-jawed and decisive to overacting and hammy and everything in between. Personally, I like to think that this shows the character's growth through the 30-odd years we saw him portrayed, although I'm sure the reality has more to do with Shatner's whims at the time he was reading each script. Regardless, what we have is a character that is either (a) extraordinarily multifaceted or (b) horribly inconsistent, and yet a character we recognize as singularly "Kirk" regardless, since each of these manifestations of his character were portrayed by Shatner. A new actor, however, cannot possibly give us every facet of the Kirk personality that was invented by Shatner, certainly not in one movie, otherwise he'd be switching from stern ship's captain to playboy-esque womanizer to melodramatic speechmaker to tongue-in-cheek comedian to KHAAAAAN! every five minutes. Hence, no actor can present to us the entirety of the Kirk that we all understand. What a new actor must do, then, in my opinion, is to comprehend how Kirk's character grew, why he changed and became the different character he was at different times, and then accurately predict what Kirk's personality was like before he became the Kirk we knew. This would have to be a new take on Kirk's character, and yet one we can accept as having the potential to become the Kirk we know (or at least, the the potential to become the Kirk presented by Shatner in the first few episodes of TOS). Not an easy task.