Zbigniew Cybulski plays one of the young soldiers who we see in the opening of the film as they gun down a couple men in car who they assume to be their target. Cybulski in this early scene plays Maciek as simply an energetic young solider who gets a thrill and almost some glee out of shooting the men and following the orders of his superior. The men quickly retreat from the scene leaving others to discover that the men were not the communists. The rest of the film depicts as Maciek and his superior wait as they are slowly given the chance once again to make an attempt on the life of the communist. In the mean time though Maciek is allowed some time to think of things over and is allowed to mind things other than what his current mission is.
Cybulski became known as the Polish James Dean after this film because of the way in which he plays Maciek, that unfortunately also became prophetic since Cybulski also ended up dying young in an accident. Cybulski only lightly takes on the mannerisms of Dean, and is nothing like Martin Sheen's more encompassing Dean influenced work in Badlands. No Cybulski only even really shows them when Maciek is interacting with his romantic interest in the film Krystyna who is the barmaid at the hotel in which they stay at until it is time to make their move again. The direction Andrzej Wajda apparently gave Cybulski was to play the part as if his character had watched Dean's films despite that technically being impossible. The whole idea might sound pretty problematic all together, but the way Cybulski brings Dean's mannerisms into his performance actually does work.
Cybulski only does his Dean mannerisms when Maciek is trying to act cool toward Krystyna, and basically it is a bit of a put on as he actively is trying to be cool. Cybulski actually plays it entirely naturally even in its technically artificiality, as it comes off as something someone would actually do when they emulate the coolness of a movie character to try to be cool himself. Whenever Cybulski's does use the mannerisms it is only at times that make sense and even then he's pretty subtle about the whole. Did Cybulski need to bring these mannerisms into his performance? Not necessarily, but I suppose they add a little more to Maciek as a character anyway. Maciek is not always trying to be all that cool with the lady though, and in these moments is where Cybulski's creates the arc that Maciek goes through during the course of the film.
Maciek's relationship with Krystyna causes changes in him as Maciek basically sees that there is more to life than his current mission. Cybulski in his early scenes was cool but seemed defined by his actions, as he spends time with Krystyna Cybulski shows Maciek as a man who starts to gain substance. He rather effectively eases back on the cool, to begin to show some genuine growth in the man as Maciek sees there is more to life. Cybulski eases very nicely into Maciek's changes as he portrays it as more of a realization than a revelation. Later on when Maciek kills again Cybulski earns the change in Maciek. In the new killing Maciek loses that extreme enthusiasm that was so pronounced the first time, and is rather powerful as he shows that the new experiences in Maciek has made it so his enthusiasm of the kill simply is impossible now.
The only problem with this performance is that it is limited due to the nature of the film. This is absolutely Andrzej Wajda's film as the film is not simply about Maciek's journey or the assassination. Andrzej Wajda takes a considerable time examining the whole atmosphere of the setting and even fairly minor characters are their due. Cybulski I would say is only barely lead as the film takes its time with all the other characters, making it so Maciek's transformation is one facet of the larger canvas of the film. Zbigniew Cybulski deserves credit for giving a compelling performance with what he does have and makes Maciek's story believable. Cybulski's performance is overshadowed by the film as a whole, but it is definitely a good performance that adds to the strength of the film.