Omar Sharif has a difficult task which is to carry this over three hour long epic on his shoulders even with his character being only the most common man at times. Sharif really takes this fact in stride though, and does not try to be anything more than what Zhivago should be. Funny enough originally Peter O'Toole was the first choice to portray Zhivago, and the fact that O'Toole turned it down was certainly a good thing. One characteristic that seems clear in Zhivago is meekness, and if there is one thing that Peter O'Toole most certainly is not is meek. Sharif though never tries to be more than what Zhivago should be and makes the most of his character despite being such a meek man.
Omar Sharif gives a very understated performance in the lead role that stresses the fact that Zhivago is not an extraordinary man by any measure. Rather just a man trying to live his life particularly early on in the film as he goes through what is expected of him marrying the woman he was posed to marrying finding a properly supportive profession as a doctor, even though he desires to be remembered as a poet, and watching all the events around him only as an observer. Sharif is very good at being this average man because he so honestly portrays the part. He does not try to fancy up what does not need to be, there is a life in this man, but only really a life that desires to live not much more than that.
Sharif effectively reflects a successful, but in many ways, average man when it comes to viewing the changes in Russia that come from the revolution. Sharif shows his reactions, at least at first, to be mostly astonishment at what is going on around as Yuri's interest never really is the revolution at hand. He just is a bystander and he only reacts to the various occurrences taking place in Russia never once taking action to implement any sort of change. When he faced with losing most of his house to revolutionary changes he acts almost slight bemusement at the idea, Sharif showing that Yuri is quite good in taking it in stride at first. The changes do slowly wear on him such as when the state takes more and more of him.
Sharif is good in showing the way that even a man who tries to take things for the best even at the worst can't deal with the suffering caused by all of the traumatic events in Russia. Sharif still does not show that Yuri changes his nature by any means he is still he same modest man, but there is a growing sadness is in performance. It is all very low key and mostly involves small quiet gestures by Sharif, but he is quite effective as showing the emotions of a sane man in an insane world. For example his scene with Tom Courtenay's Strelnikov, Courtenay of course is electrifying in the scene with his intense depiction of the dehumanization of a man, but Sharif is just as important in showing the completely human and heartbreaking reaction as he shows Yuri's sadness and disbelief over the change in the man.
Of course I should mention that a pivotal part of the film is the central love story between Yuri and Laura (Julie Christie). Now Sharif here is quite excellent as he builds up the love of Yuri, and again Sharif does not overplay this. Early in the film he is good as we see him notice her, and Sharif conveys an unshakable feeling for the woman who Yuri does not even know who she is. Of course they do meet, and Sharif is very good in showing the love that slowly grows with him. He never comes out with it obviously, and at first they do not act upon since they are both married. Even when they finally do have an affair Sharif still stays true to the character, and leaves much unsaid. Sharif and Christie make their relationship something special because both stay subtle and make it far more truthful than if they had conveyed the emotions in a broader fashion.
It would have been very easy for many actors to try and portray the character in a different more constantly active, and could frankly have been trying to steal scenes away from the likes of Rod Steiger and Tom Courtenay. That would have been the wrong approach though. In one scene in the film Zhivago states that he would rather just live life rather than try to be the surgeon for the world, and that is very much Sharif's portrayal. He does not try to control any scene of the film with his performance, but rather he stands firm in his realistic portrayal of an ordinary man in an extraordinary situation. This really is not the usual performance for an epic, but Sharif makes the most of his character and successfully carries the film.