This is an interesting question. It would be simple to imagine that one can evaluate the behaviour of a character in a play based upon simply reading the script, but the behaviour of a character in a play is not a complete event until an actor takes up the part and performs in on stage. This is because the actor's choices -- how loud or softly he speaks his lines, whether he is moved to tears or not, where he stands or sits on stage and in what proximity to the other characters in the scene, etc. -- all of the actor's choices in combination with the lines that they speak create the behaviour of the character.
One can evaluate how "realistic" the lines that Shakespeare has written seem to be, but this shouldn't be confused with evaluating the character's behaviour. This is one of the reasons why reading a play can be a tricky thing to do. It is tempting to imagine that the text itself is 100% of the play, but scripts aren't meant to provide the action of the characters. The action, or behaviour, is chosen by the actors performing the play. And this element, omitted when one simply reads the text, is crucial to a complete understanding of any play.
Let's look at the text that comes in response to the news of Macduff's family's murder. When, in Act IV, scene iii, Ross begins to tell Macduff of the murders, Malcolm speaks to Macduff:
What man! Ne'er pull your hat upon your brows.
Give sorrow words. The greif that does not speak
Whispers the o'er-fraught heart and bids it break.
These lines are significant, even though they are not spoken by Macduff. Shakespeare has done something here he often does. He's given lines to one character that suggest the behaviour of another. Malcolm cautions Macduff not to hide silently beneath his hat, but to speak out his feelings.
Macduff, at first, has short, seemingly unemotional responses:
My children too?
And I must from thence.
My wife kill'd too?
But his emotions seem to grow as the realization really takes hold:
. . .All my pretty ones?
Did you say all? O, hell-kite! All?
What, all my pretty chickens and their dam
At one feel swoop?
It is hard to know the behaviour of Macduff during these lines, but the text indicates that he is having trouble taking the news in, since he repeats the information as questions.
All of this seems very realistic to me -- to first be in such a state of shock that one cannot say a thing, and then to only be able to question the information. Macduff speaks as a man who has received overwhelming news and simply cannot take it in. He says to Malcolm that he must "feel it as a man," which suggests that he cannot consider revenge until he has really taken in the enormity of his loss. Shakespeare has created a very realistic sequence of text for Macduff, I think, a sequence that displays the journey it must be to really take in being blindsided by the news that one's entire family has been murdered.