How does Hamlet behave before the play begins?
We have absolutely no direct textual evidence to answer this question, but we can draw some inferences make some educated, logical guesses.
1. Hamlet loved and admired his father and his parents' marriage. He makes references to both of these things throughout the early parts of the play.
2. He was a student at Wittenburg, which is a very traditional conservative school in Germany. We could probably assume he did well because he seems smart and rather philosophical. He is also a bit old to still be in school if he didn't want to be there.
3. He was kind and romantic with Ophelia. She defends Hamlet's actions and speech with both her brother and her father when they talk about Hamlet's intentions.
4. He has several friends. Horatio is closer to him as evidenced by the fact that he came to the funeral and the quick wedding, whereas Rosencrantz and Guildenstern have only just arrived at the King's bidding. He shows that he has a sense of humor with his friends when he first meets up with Ros and Guil in Act 2.
5. He is interested in the theatre and acting. He knows the players personally, and he is able to quote a long section of a speech from a play that he says he only saw them perform one time. He knew the plot of a play that he could them quickly adapt to suit his plan to use the play to catch Claudius's guilt.
6. He is religious. He comments on the morality of others and directly says that he has contemplated suicide but won't act because it is against God's laws.